This error tends to happens when you have a user in the cs_users table who's not in the aspnet_users / aspnet_membership tables. Query aspnet_Applications and check the application names. Cross reference that with aspnet_users and make sure they match. Also, check the web.config in the "Membership node".
select * from cs_Users where MembershipID not in (select UserId from aspnet_Membership)
select * from aspnet_Membership where UserId not in (select MembershipID from cs_Users)
It's also possible the issue is simply a mismatch between usernames between the cs_Users table and aspnet_Users. If you still need assistance, you can open a support ticket and I can send you a SQL query to help identify orphaned users, as well as one to fix them.