Turned out the admispconfig user can't edit /etc/passwd and /etc/group if there's no empty line at the end.... Well... I think so. I restored both files before and probably edited them, removing the last empty line.
After adding an empty line and running /etc/init.d/ispconfig_server restart I had some more problems (In the admin gui i emptied the recyclebin, which removed several old deleted sites), because apache couldn't get up (cannot open the logfiles of several web[n] sites).
I then vimmed into /etc/apache2/vhos/Vhosts_ispconfig.conf and deleted the Virtualhosts my /var/apache2/error.log was complainting about.
Finally after restarting the ispconfig_server again, it ran and all sites are up! Adding a new user is also possible.
Tonight I'll try to add a new site, to see what happens... If that doesn't work as it should, I'll post back (to myself ;-) ) again.
I hope this thread will help someone having the same probs i had!