Sorry you are having trouble. I know the system requires the e-mail address and password to match the forum membership details. Offhand, I can't see what is causing the problem.
I think the quickest & easiest solution may be to buy a pass with a new username. Then if you want to change it to match your original username, just contact
One Click Pass support and they can help.
Note: You will need to login to the forum with your One Click Pass username for full access.