Just wanted to let you know that this is something that is being looked into. I know its very frustrating, and we do thank you for your continued patience while we look to rectify this matter.
Apologies, no update at this time.
Some users have had luck joining a clan from the console or mobile app, or a different web browser - like Chrome or Firefox. Additionally, you might try un-linking, and re-linking your gamertag to Elite, or creating an all new Elite account with a new email address, and linking your gamertag.
Thanks for your continued patience while we look into this - we do realize this is very frustrating for all of you!
I have this same problem, and so does my clan leader, when he tries to accept me it gives him an error, and when I try to accept the invite it gives me an error.
I have tried on the console elite app and on the elite website.