if I understanded correctly your PM, you gave me link to the pure punbb installation (without any extensions). Unfortunately the error still occurs. Not good, not good... I made some tests, and I noticed, that the error appears only when posting new message - editing is working perfectly fine.
post.php - post / preview => error
edit.php - post / preview => no problems...
In this case.... i have no idea, what is going on, maybe some freaky server settings (like mod_rewrite)? Let's try to change the name of the file from post.php to something else like "pretty_name.php", and you need also to correct the values in include/url/Defaut/forum_urls.php - change each line that contains "post.php" to "pretty_name.php". Now, when you will try to add new post, you will be redirected to page pretty_name.php instead of post.php.
If this doesn't help, then maybe there is really some problems in the source code There is no extensions, so it must be something inside post.php. Let's concentrate on preview - this operation is shorter, so there is less places where the error can hide
in the lines 345 - 382 there is following code fragment:
// If preview selected and there are no errors
if (isset($_POST['preview']) && empty($errors))
.... // here was some code that I cutted out
try to delete this fragment from file, and check if error dissapears after pressing preview button? If yes, you can restore this code piece by piece and check, what line is causing the problem.
Tell us what were the results of the hints I gave you, then I will continue.
If you want to contact me quickly - send e-mail, not PM.
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