Finally it is working! Thank you very much for your awesome help guys!
What I did?
- Resetted the whole Ubuntu Touch device to factory defaults
(probably some Dekko fragments of UT 15.04 have been hanging around)
- Installed a virgin Dekko from OpenStore
- Setup all user profiles one after each other
Finally the wizard also displayed the magic page where one can specify server settings by hand. This page has never appeared before...
Furthermore, I see the message "Your acount has been set up successfully" or something similar after completion - that also did never happen before...
Thanks a lot for your kind help!