Wow, thanks a lot for your help! I have been in contact with hosteurope in the meantime and apparently they don't allow changes in the nameserver. Apparently, the only way would be through setting a CNAME, but I am not sure how to do this. Do I have to do this through hosteurope?
I would be very grateful if you could edit my dns settings, I'm sure I made some horrible mistakes there.
Best,
Jochen