Hello there.

OpenProject doesn't send any emails (invite, etc), although Send a test email works fine.

I tried OpenProject installed as Docker container and also tried direct installation. Ubuntu 18.04.

How can I solve it?

Solved, after I restarted host server (restart docker container didn't help).

Thank you mate! The same problem occures on a manual installation with Debian Buster 10.3.

It had been grinding my gears half the day. Couldn't believe the solution is as easy as restarting the virtual machine.