Thanks. That seems to have soled the issue.
It's a shame that there is no hint about this anywhere to be found

In times when plugins often break crashing the whole app user tend to disable many plugins. It would be very helpful to get warned about the consequences about this kind of plugin-disabling...
Another questions (off topic):
How to properly upload and link/reference images/screen shots inside a post? I kind of managed to do so and the image is being shown when I'm logged in but the image is not being shown to a random not logged in forum user. Wtf?
How to improve the captcha? I'm having big trouble identifying the letters in the captcha to the point where the forum becomes almost unusable to me. The "listen to letters" function seems broken as well. So submitting a post to the forum needs several attempts (8 or so) and is very annoying.
This really needs to change.