[KHALIK]

Followed the following instructions;

Tested:
Admin CP -> Styles & Templates -> Style Manager -> Common Templates (in the menu) -> headinclude
** problem still remains. <---

Tested:
Admin CP -> Styles & Templates -> Style Manager -> [Add New Style]
** problem still remains. <---

Tested:
If you still have the problem then try disabling your plugins:
Admin CP -> vBulletin Options -> Plugin/Hook System

** define('DISABLE_HOOKS', true); // completely disabled all hooks
** problem still remains. <---

Tested: Also run a file check to look for customized files. Upload the original files if any problems are reported:

Admin CP -> Maintenance -> Diagnostics -> Suspect File Versions
*** removed all suspect files and problem still remains <---

The problem is still there, same error message.