FileMaker will put your current UserName, as specified in preferences in as the default account name. That's why you see your user name appearing in the dialog.
Try opening the file this way:
enter Admin as the account name. Leave the password box empty and try to open the file. If that works, go to file options, click the "log in using" check box, then enter Admin as the account name and leave the password box empty. This is the standard "default" setting for new FileMaker files.
If that does not work, you might try opening the original 5.5 copy and check document preferences for this same password setting. Perhaps you set the original database to auto-enter a password and you've forgotten that detail.