I finally could fix my scope, the problem with the serial connection was the UART I used (it was one with a micro USB connector, not sure if it was the type or it was just broken). I tried with another one (with a USB A connector) and the sonnection established immediately.
Now all I had to do was to delete all the files in /dso/app/etc/aslf (as described somewhere up in this thread) and the scope boots again as before. Seems to be a bug in this scope, but once you know its easy to fix.
I also updated the system.inf file to get 250 Mhz, works also fine, and afterwards I installed an upgrade to 1.1.2, no problem at all. So I think its not necessary to hack the eeprom as well.
By the way opening the scope I broke the power push button, and ordered 6 new ones instead of 2 (they came in sets of 3), so if anybody needs one just write me (if you are not too far away from me - Austria, of course)