Update:

Thanks for the ideas guys. Unfortunately, neither I nor my locksmith could get them to work... After 1.5 hours the locksmith gave up and cut open the bike rack. He then took my bike to his shop and promises me that he can use some kind of grinding machine to hack the lock off by tomorrow morning.

The frame is already way scratched up. Hope the bike still works OK. I'm not looking forward to seeing his bill or the cost or repairing/replacing the bike rack... I'll let you know what happens.