Error when accepting a campaign invite
In rare circumstances, invitations sent to a user's email address will not match with the user, making it impossible for them to accept the invitation.
The possible errors you might receive are "this invitation is not for you" or an error 500.
The solution is easy, however. If this is happening, the GM should simply remove the invitation, then re-send it to the username, not email. Sending an invitation to a user's username always works.