Thread:Project:Support desk/Special:UserLogin error 500 after upgrade to 1.18.5/reply

From which version did you upgrade?

I don't think some defines; should change much. If you want to disable PHPBB/Users Integration, you should disable the require_once-line, which includes this extension.

Apart from that you should find more information in the Apache error log and maybe also in the PHP error log. Check what they tell you!