Cannot synchronize RBS20 to RBR40

Hi guys,

I have a RBR40 router and two RBS40 satellites wich I bought as package ~2 years ago. Now I wanted to add a new satellite and saw that - at least on Amazon - you can't buy the RBS40 anymore. Therefore I purchased a RBS20 but I'm not able to sync it to my RBR40 router. I tried several locations (near and far away from the router) but always the same result: After clicking on "Add new satellite" (don't know the right labels in english because my UI is in german), pressing on the "Sync" button on the RBS20 and then on "Next" in the RBR40 web ui, it tells me that this process can take up to 6 minutes. During this time the RBS20 keeps pulsing white light and after ~3 minutes the webui says that "configuration of the satellite failed" (while the RBS20 is still pulsing white light).

I also tried to factory reset the RBS20 although that this should be needed since this is an entirely new device. 

Is the RBS20 not compatible to the RBR40? Any other ideas?