I have had this problem. Off the top of my head:
FM6 does not have a place to enter an account number that is associated with the password. On Later versions of FM there is a place to enter the account number and then the password. The problem arises when the account name on the later file versions does not match the account name on the earlier versions.
Simple solution if it works: In FM6 look at the name that is above the field for the password. Use that name as the account name for the updated file and the associated password.
If not: In the FM6 Access Priveledges what you may be able do is to make the file so it does not require a password to open or you can find in the same area the account name that was associated with the password. Or it may be the name of the file.
Others may be able to add to this. I would have to find an old FM6 to go further. The above illustrates why you have the problem