Is it possible that email notifications are not working in the current (12.1.5) release?
I receive the test mail, new users receive a mail, but any other interaction with tickets just do not send an email.
I already set up a completely new instance, but its simply not working and I can't find out why.

Initially I got this problem with the upgrade from v10 to v12. But as already mentioned a complete reinstall with a clean demo database also doesn't send notification mails.