unable to add family members to premium account

[feasco]

When my Spouse tries to accept the family premium invitation, the following notification appears: Oeps! Er is iets fout gegaan. Probeer het opnieuw.

(Oops! Something went wrong. Please try again.)

I've tried several laptops and different internet connections

until 2 weeks ago everything was fine. at that moment she was asked to verify her adress. the website never accepted the input and as a consequence she was thrown of the account.

is there an other way to add her?

may i also suggest some more informative error messages.