Jeroen Massar:
Now, I have to report to Hetzner, I will tell them that I'm running a TOR exit node in "restricted" mode, but how can I defend myself, I am not sure that my "restricted node" and "given the nature of the TOR network" arguments will convinced them the Hetzner dudes.
You cannot "defend" yourself. There is no way for anybody to be able to claim that it was you, not you, or somebody else. That is the bad thing about an exit. You are responsible what happens from that IP.
Sorry but the last statement is wrong in many jurisdictions: https://trac.torproject.org/projects/tor/wiki/doc/TorExitGuidelines#Legal
For Germany, see TMG §8 and §15.