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Surely it must be a setting of some sort rather than code? If it used to work, but now it doesn't, and yours works using the same post.php file (the original one). I was going to say can I have a copy of yours, but it will just be the same as the Core NewBB post.php file that I have already used.
Note that all this started after I turned 'register_globals = on' in php.ini to get the ZenTrack module to work. Then this happened to I changed php.ini back to how it was and re-started the web service, but the problem persits.
A mystery.