The insatllscript failed due to insuffient/wrong rights on files. reqheap needs 777 permission, but server allows maximum 775.
The host has phpsuexec installaed. So the needed rights were not reached.
It resulted in a 500 internal server error as the permission was changed back.
'500 Server Error
A misconfiguration on the server caused a hiccup. Check the server logs, fix the problem, then try again. '