With FP7 files there is now both account names and passwords. And the passwords are now case sensitive when they were not in FP5 files.
During conversion, FileMaker will take any existing password and create an account where the account name is also the password. So you should be able to enter the password as both the account name and then also the password to open the file. Then you can go to the correct option in the File Menu to change your account name so that it no longer matches the password.
You may also find that you can open the newly converted file with an account name of "admin" with the password box left empty.