I could suggest then to try changing identifier type to another for example Address (I am not sure if Fritzbox supports IKEv2), then mode and your identifier values could be also switched. Also, in the host and IP address/subnet mask you have entered Fritzbox address, right?
It would be interesting to analyze the logs of RUT850 when you try to connect to fritzbox IPsec. Logread output may identify where is the problem.