Apparently I just finished soemthing impossible: I moved from an "old server" running ISPConfig 2.2.29 to a "new server" running 2.2.37.
I had to do an emergency recovery of a crashed server. I was able to backup all SQL databases in a dump and to perform an backup as described here http://www.howtoforge.com/forums/showthread.php?t=2717
Afterwards I installed ISPConfig 2.2.37 following http://www.howtoforge.com/perfect-se...64-ispconfig-2
Having done this I restored all data an got an error message "The requested document type does not exist." each time I tried to change any user or website settings inside ISPConfig. So I reinstalled ISPConfig 2.2.37 and the problem was solved.
Sometimes one has to post a success story just to say thank you to the developers and all forum contributors!