Update:
Now running 2.6.3-t and everything seems to work so far, except WLAN/WiFi!
When I try to enable it, it gets disabled again after a few seconds, no networks are discovered.
Unfortunately WiFi this is the most important thing in my case 
.
I remember there was nearly exactly the same issue quite some time ago when I upgraded from 1.3 to 1.4 or 1.4 to 1.5 (or so). But that was a bug in eOS.
Any idea what I can do?
Is there a possibility to go back to 2.4.1-s (loosing all data is OK because I have a backup)? If, so, what exactly do I have to do? Installing 2.4.1-s via sideloading does not work anymore due to new Security Patch Level.