Resolving Email Address Conflict with User Ownership Issues

Hi, I used to have a user with email adres x. Then that user was removed and I log in with a different email address now. Both are admins. However the first user is apparently still the owner of some connections that I cannot modify now. So I tried to create the original user again, I can send the invite but when I try to accept it, I get an error 'email address not available'. How can I solve this?