New RBS50 addon - continuously blinks white, never goes solid white

I already have an RBR50 and RBS50 (bought as a kit) setup and working in AP mode, I updated both to a recent firmware version.

I have now bought an additional RBS50. When I plug it in, it flashes white continuously and never goes solid white, even after leaving it overnight.

If I go to the "add satelite" section of the orbilogin.com system, and press Sync on the new RBS50 and then click the Sync button the page, it waits a short while and then says that the satellite add failed.

The new RBS50 also doesn't appear in the attached devices or attached satellites list.

I've tried putting the RBS50 in various locations, both near and far from the router and powered it on and off (as well as power cycling the other devices), and also did a factory reset to no avail. It just sits there permanently pulsing white.