External member can't sign in

Hi everyone, 

I have set up a Sharepoint but some of the members have trouble signing because it says 'we currently cannot accept your invitation. Try again later' despite me being able to see her as a member. I have tried sending new invitations or using different emails but we end up at the same point again. She has had access before and just recently had trouble signing in which is werid.

I hope someone can help!