I updated with 2.2.29 and now it works!
Let me just shortly remark, that now after updating I have to use https to connect to my :81 ISPconfig, before I had it unencrypted, but this is also ok, so it forces me to more security
At this time let me say thanks to you for your great support and howtos and of course for developing ISPConfig in such a perfect way. I use this software now for nearly two years in a production environment and it never disappointed me!
Keep on doing the great work! If 3.0 is stable and the update/migration procedure will be not too strange, I'm sure I will do the same good experiences as I did in 2.x.