Have you added any mods? I am just thinking of ways that the file would get altered, assuming it did anyways.
What you can try is to download the full install from phpbb's website and look for viewtopic.php and upload that file to your site make sure you place it in the same directory that it is now.
If you have any mods, you will then need to download the mods and see if any of them alter viewtopic.php if they do, then you will need to follow the directions carefully to make sure that file will still work once your done editing it.
you may want to post your question on phpbb's forum as odds are good they know exactly what that error message means.