When we post a form with PHP, some errors maybe return, such as: 500 SERVER Error, or:

Forbidden
You don't have permission to access /post.php on this server.Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Here are some explains may help:

We had installed a software to block attack against the web-server which caused the trouble for you.We have tweaked that to allow the URL format.

A similar reason same as the above explain, they will redirect your post, when your post includes some "security" words, such as: "inc","curl".

Further more optional reasons:

In the control panel of hosting, the PHP scripts doesn't need special permissions, but mod_php is configured in safe mode, so if I need PHP-CGI scripts or applications that install something, You need permissions (770). Besides, need to create a .htaccess file with the command: Options ExecCGI.

mod_security:
ModSecurityTM is an open source intrusion detection and prevention engine for web applications (or a web application firewall). Operating as an Apache Web server module or standalone, the purpose of ModSecurity is to increase web application security, protecting web applications from known and unknown attacks.

While mod_security can be a very powerful tool, misconfigured or overly strict rule sets can interfere with vBulletins Operation. Below for Apache users you can use an htaccess file and add a specific rule to disable mod_security.

Make or edit your forum .htaccess file and add the following code
Code:

< IfModule mod_security.c >
SecFilterEngine Off
SecFilterScanPOST Off
< /IfModule>

or

< IfModule mod_security.c>
SecFilterInheritance Off
< /IfModule>

place this file in your problem directory.
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