Hi Jan, 

Welcome to the Box Community, I'm happy to help!

I understand that you cannot access a Shared folder you have been invited to. If a user is invited to collaborate on a folder and they do not accept this collaboration invitation (from the email notification received or the Pending Items Manager in the user's Box account) then the collaborative folder will not appear in the user's Box account until the collaboration invitation has been accepted. If a user does not accept an invitation within 30 days, the invitation is removed.

You may need to reach out to the owner of the folder and request if they can re-invite you to the Shared folder again.

To learn more about inviting collaborators, please take a look at this article: Inviting Collaborators

All the Best, 

Why is an invitee to my Box folder is being asked for a credit card.

I keep ACCEPTING the invites but NOTHING happens.  NOTHING. NADA. ZILCH. NOTHING