whenever you have mods installed, and messages/users are created, the structure in the MySQL tables are usually a lot different than if you had an unmodded version of phpbb
so now that you have the unmodded version, and are trying to backup information that was created in a modded version, you’ll most likely never get it to work unless..
1) you re-install the mods and then try the backup
2) have someone (with A LOT of time) sit there and edit the backup file in notepad and edit the structure of each message and username to fit the unmodded version
i know its not exactly good news, but hope it helps