Thank you for your time Derelic. I looked at the interfce settings and found out, that I had left the subnet-setting for the interface at /32 instead of changing it to /24. Maybe this setting is bit confusing, because if I should set a single IP-address for the router interface and not a subnet. I suggest giving this setting a separate line or at least a caption "subnet size" or something similar. Anyway - excuse my dumbness. Best, Karl

@grimm-spector Exactly, it will work just fine :)