I have used OpenProject for almost 3 months. Everything went well until a few hours ago, the server could not be opened and down. I've check the error.log from apache log and got this error:

[Mon Nov 26 13:25:04.825955 2018] [mpm_prefork:notice] [pid 9593] AH00163: Apache/2.4.18 (Ubuntu) SVN/1.9.3 mod_perl/2.0.9 Perl/v5.22.1 configured -- resuming normal operations
[Mon Nov 26 13:25:04.826011 2018] [core:notice] [pid 9593] AH00094: Command line: '/usr/sbin/apache2'
[Mon Nov 26 13:25:04.885761 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/qa.php' not found or unable to stat
[Mon Nov 26 13:25:05.067996 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/Ss.php' not found or unable to stat
[Mon Nov 26 13:25:05.932684 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/xxx.php' not found or unable to stat
[Mon Nov 26 13:25:06.741202 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/92.php' not found or unable to stat
[Mon Nov 26 13:25:06.919831 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/z.php' not found or unable to stat
[Mon Nov 26 13:25:07.097863 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/x.php' not found or unable to stat
[Mon Nov 26 13:25:07.275737 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/dexgp.php' not found or unable to stat
[Mon Nov 26 13:25:07.454655 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/nuoxi.php' not found or unable to stat
[Mon Nov 26 13:25:07.987479 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/godkey.php' not found or unable to stat
[Mon Nov 26 13:25:08.165641 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/okokok.php' not found or unable to stat
[Mon Nov 26 13:25:08.343602 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/erwa.php' not found or unable to stat
[Mon Nov 26 13:25:08.522176 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/pma.php' not found or unable to stat
[Mon Nov 26 13:25:08.700326 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/ruyi.php' not found or unable to stat
[Mon Nov 26 13:25:08.878647 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/51314.php' not found or unable to stat
[Mon Nov 26 13:25:09.057040 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/5201314.php' not found or unable to stat
[Mon Nov 26 13:25:09.818005 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/fusheng.php' not found or unable to stat
[Mon Nov 26 13:25:10.694170 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/general.php' not found or unable to stat
[Mon Nov 26 13:25:10.872234 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/repeat.php' not found or unable to stat
[Mon Nov 26 13:25:11.050302 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/ldw.php' not found or unable to stat
[Mon Nov 26 13:25:11.228271 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/api.php' not found or unable to stat
[Mon Nov 26 13:25:11.406301 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/s1.php' not found or unable to stat
[Mon Nov 26 13:25:11.584452 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/xiaodai.php' not found or unable to stat
[Mon Nov 26 13:25:11.762564 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/hello.php' not found or unable to stat
[Mon Nov 26 13:25:11.940505 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/hello.php' not found or unable to stat
[Mon Nov 26 13:25:12.118383 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/admn.php' not found or unable to stat
[Mon Nov 26 13:25:12.297293 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/hell.php' not found or unable to stat
[Mon Nov 26 13:25:12.475931 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/xp.php' not found or unable to stat
[Mon Nov 26 13:25:12.653937 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/1.php' not found or unable to stat
[Mon Nov 26 13:25:13.175490 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/2.php' not found or unable to stat
[Mon Nov 26 13:25:14.474696 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/p.php' not found or unable to stat
[Mon Nov 26 13:25:14.744340 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/1.php' not found or unable to stat
[Mon Nov 26 13:25:14.922196 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/a.php' not found or unable to stat
[Mon Nov 26 13:25:15.100108 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/m.php' not found or unable to stat
[Mon Nov 26 13:25:15.278183 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/conf.php' not found or unable to stat
[Mon Nov 26 13:25:15.456293 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/123.php' not found or unable to stat
[Mon Nov 26 13:25:15.634153 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/HX.php' not found or unable to stat
[Mon Nov 26 13:25:15.812398 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/diy.php' not found or unable to stat
[Mon Nov 26 13:25:15.990251 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/666.php' not found or unable to stat
[Mon Nov 26 13:25:16.168151 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/777.php' not found or unable to stat
[Mon Nov 26 13:25:16.346137 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/qwq.php' not found or unable to stat
[Mon Nov 26 13:25:16.524096 2018] [authz_core:error] [pid 18367] [client 205.164.63.82:7196] AH01630: client denied by server configuration: /opt/openproject/public/.php
[Mon Nov 26 13:25:16.702612 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/infos.php' not found or unable to stat
[Mon Nov 26 13:25:16.881597 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/x.php' not found or unable to stat
[Mon Nov 26 13:25:17.062433 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/htfr.php' not found or unable to stat
[Mon Nov 26 13:25:18.104018 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/zzk.php' not found or unable to stat
[Mon Nov 26 13:25:18.750500 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/toor.php' not found or unable to stat
[Mon Nov 26 13:25:18.928222 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/uu.php' not found or unable to stat
[Mon Nov 26 13:25:19.106336 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/aa.php' not found or unable to stat
[Mon Nov 26 13:25:19.284403 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/wb.php' not found or unable to stat
[Mon Nov 26 13:25:19.463470 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/yj.php' not found or unable to stat
[Mon Nov 26 13:25:19.641810 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/z.php' not found or unable to stat
[Mon Nov 26 13:25:19.820304 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/7.php' not found or unable to stat
[Mon Nov 26 13:25:19.998306 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/xiaoma.php' not found or unable to stat
[Mon Nov 26 13:25:20.176354 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/xiaomae.php' not found or unable to stat
[Mon Nov 26 13:25:20.676604 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/xiaomar.php' not found or unable to stat
[Mon Nov 26 13:25:20.855575 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/qq.php' not found or unable to stat
[Mon Nov 26 13:25:21.473295 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/data.php' not found or unable to stat
[Mon Nov 26 13:25:22.632831 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/log.php' not found or unable to stat
[Mon Nov 26 13:25:22.810818 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/fack.php' not found or unable to stat
[Mon Nov 26 13:25:22.988555 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/angge.php' not found or unable to stat
[Mon Nov 26 13:25:23.166621 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/cxfm666.php' not found or unable to stat
[Mon Nov 26 13:25:23.344960 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/db.php' not found or unable to stat
[Mon Nov 26 13:25:23.522952 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/hacly.php' not found or unable to stat
[Mon Nov 26 13:25:23.700905 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/xiaomo.php' not found or unable to stat
[Mon Nov 26 13:25:23.878903 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/xiaoyu.php' not found or unable to stat
[Mon Nov 26 13:25:24.057391 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/xiaohei.php' not found or unable to stat
[Mon Nov 26 13:25:24.236970 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/j.php' not found or unable to stat
[Mon Nov 26 13:25:24.415168 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/qq5262.php' not found or unable to stat
[Mon Nov 26 13:25:24.593185 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/MCLi.php' not found or unable to stat
[Mon Nov 26 13:25:24.772065 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/MCLi.php' not found or unable to stat
[Mon Nov 26 13:25:24.952193 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/51.php' not found or unable to stat
[Mon Nov 26 13:25:26.223909 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/cadre.php' not found or unable to stat
[Mon Nov 26 13:25:26.743291 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/mm.php' not found or unable to stat
[Mon Nov 26 13:25:26.921097 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/test.php' not found or unable to stat
[Mon Nov 26 13:25:27.099023 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/1q.php' not found or unable to stat
[Mon Nov 26 13:25:27.277070 2018] [:error] [pid 18367] [client 205.164.63.82:7196] script '/opt/openproject/public/index.php' not found or unable to stat
[Mon Nov 26 13:40:46.838782 2018] [autoindex:error] [pid 18366] [client 211.117.199.91:40501] AH01276: Cannot serve directory /opt/openproject/public/: No matching DirectoryIndex (index.html,index.cgi,index.pl,index.php,index.xhtml,index.htm) found, and server-generated directory index forbidden by Options directive
[Mon Nov 26 14:05:05.114975 2018] [autoindex:error] [pid 18371] [client 195.154.61.206:56781] AH01276: Cannot serve directory /opt/openproject/public/: No matching DirectoryIndex (index.html,index.cgi,index.pl,index.php,index.xhtml,index.htm) found, and server-generated directory index forbidden by Options directive
[Mon Nov 26 15:10:11.309356 2018] [autoindex:error] [pid 18370] [client 35.168.20.163:33524] AH01276: Cannot serve directory /opt/openproject/public/: No matching DirectoryIndex (index.html,index.cgi,index.pl,index.php,index.xhtml,index.htm) found, and server-generated directory index forbidden by Options directive
[Mon Nov 26 16:22:57.164932 2018] [autoindex:error] [pid 18366] [client 37.134.249.51:46317] AH01276: Cannot serve directory /opt/openproject/public/: No matching DirectoryIndex (index.html,index.cgi,index.pl,index.php,index.xhtml,index.htm) found, and server-generated directory index forbidden by Options directive
[Mon Nov 26 17:28:44.980886 2018] [autoindex:error] [pid 18595] [client 203.207.57.40:60184] AH01276: Cannot serve directory /opt/openproject/public/: No matching DirectoryIndex (index.html,index.cgi,index.pl,index.php,index.xhtml,index.htm) found, and server-generated directory index forbidden by Options directive
[Mon Nov 26 18:40:37.260879 2018] [autoindex:error] [pid 18595] [client 124.244.254.198:60757] AH01276: Cannot serve directory /opt/openproject/public/: No matching DirectoryIndex (index.html,index.cgi,index.pl,index.php,index.xhtml,index.htm) found, and server-generated directory index forbidden by Options directive
[Mon Nov 26 18:45:01.738291 2018] [mpm_prefork:notice] [pid 9593] AH00169: caught SIGTERM, shutting down
[Mon Nov 26 18:45:03.077876 2018] [so:warn] [pid 22828] AH01574: module dav_module is already loaded, skipping
[Mon Nov 26 18:45:03.202207 2018] [mpm_prefork:notice] [pid 22829] AH00163: Apache/2.4.18 (Ubuntu) SVN/1.9.3 mod_perl/2.0.9 Perl/v5.22.1 configured -- resuming normal operations
[Mon Nov 26 18:45:03.202244 2018] [core:notice] [pid 22829] AH00094: Command line: '/usr/sbin/apache2'

I don't touch anything on that server as long as OpenProject is running, and that's why I ask this question here. What should I do?