It works!!! Thanks alot.
So what I did is put the public key from the fritzbox web ui and the private key from the config file under [Interface] and entered it in the general settings tab.
So there were basically three issues:
- The RUT200 resolved the endpoint address via IPv4, but the fritzbox is reachable via IPv4 only. I figured this out by a IPv6 udp port scan with a online tool
- I gave the fritzbox and the RUT the same IP address for the tunnel. They need different IP adresses
- The public and private key of the interface defined in the RUT settings need to be the keys generated by the fritzbox. The public key is displayed in the web UI, the private key is listed in the config file under [Interface]