This problem is arising mainly because of "mod_security"; a web application firewall. All the cPanel hosts use the "mod_security" feature.

So What is mod_security?
ModSecurity is an embeddable web application firewall. It provides protection from a range of attacks against web applications and allows for HTTP traffic monitoring and real-time analysis with no changes to existing infrastructure. It is also an open source project that aims to make the web application firewall technology available to everyone.

Solution to the Problem:

Add the following lines in your .htaccess file.

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