Let me see if I understood correctly (sorry, I don't speak German, but I did ask uncle google for assistance :-)):
-You have a Thunderbolt display
-Your mac does not detect any device connected to the Thunderbolt ports
-You are prompted to install a thunderbolt firmware update, but it fails to install every time
There are a few simple tests that may help clarify the situation a bit, if you can:
1) Try other Thunderbolt devices with your mac - do they work correctly?
2) Try your Thunderbolt display with another mac - does it work?
3) Start your mac in Safe Mode. Does the display work?
At this point, I am suspecting that a failure of the firmware update may have left the display in an undefined or unstable state.
You may have to take it to an authorized repair shop, but the tests above should help determine if this is the case.