404 error accepting invitation

Invite is 2 days old, have tried signing out and back in, no luck.
How can I resolve this?

GitHub invitations are only visible to the user account to whom the invitation was send. I.e. you need a GitHub account (not just a GitHub Community account) to view and accept the invitation.

In fact, all invitations have the same URL, and every invited collaborator will see his/her own invitation page, or a 404 error page in case the user is not logged in (or has no GitHub account).

Maybe your confusing the invitation to become a repository collaborator with an invitation to join GitHub — I don’t recall there are invitations of the latter type, to join GitHub you only need to create an account on https://github.com.

In other words, a GitHub account and the GitHub Community account are independent of each other (although you can sign into the latter with the former).