The family premium is not accepting the second invited member of my family, reporting a weird error, and the profile is the same, I mean, address etc..
Apologies for the late reply, but maybe I can still help :)
Have you tried to use an incognito tab, clear the cookies of your browser and use another browser or device?
When this doesn't work, you should try to use another internet connection (e.g. cellular data instead of WiFi).
Have a nice day!