which was good for mantisbt 2.2.0.

The minimum requirement - as stated in the documentation - is 5.5.x for all 2.x versions. We only started effectively enforcing that with 2.7.0.

I agree that the user-friendliness could be improved, but at the same time I believe that as a system admin you should a) RTFM and b) be able to check a system log for errors when something unexpected happens...

I agree to you and like i wrote i know what cause the error ;).

So i have done a), the readme which comes with mantis says check the system with check/index.php and this doesn't even work. So i've taken a look in the server log.
And as a programmer it pains me when software cause an error and you have to search for it instead of getting a clear message ;)
So i thought the check does what the name says and checks if the system in its configuration is compatible with mantis.

PR https://github.com/mantisbt/mantisbt/pull/1318