We use OP via docker (all-in-one-container).
( docker run -it -d -p 88:80 --name openproject --env-file /usr/bin/openproject.env -v /var/lib/openproject/pgdata:/var/openproject/pgdata -v /var/lib/openproject/assets:/var/openproject/assets openproject/community:13)

I couldn't get it working to make OP look for incoming mails via IMAP. 

The env - file is:
OPENPROJECT_HTTPS=true
OPENPROJECT_HOST__NAME=XXX.de
OPENPROJECT_SECRET_KEY_BASE=secret
IMAP_PORT=993
IMAP_HOST=xyz.kasserver.com
IMAP_USERNAME=xyz@xyz.de
IMAP_PASSWORD=secret
IMAP_ALLOW_OVERRIDE=unknown-user=accept,no_permission_check=1
IMAP_SSL=true
IMAP_SSL_VERIFICATION=true

Any syntax error? Something missing? Or not active in community version?

Which logs should be checked?

Thank you in advance!