Hi Serigne,
Fixed. Now you can create a new pages.
Probably You forgot that you didn't rename htaccess.txt to .htacces file OR it doesn't have orginal code anymore, and you set enabled "Use URL Rewriting" in Global Configuration > Site > SEO.
My suggestion is: please check content of .htaccess file - I can only guess that orginal Joomla file was was replaced (removed) by cPanel after PHP changes, it's quite popular issue on some servers. Then after all with correct file content you can enable (On) "Use URL Rewriting" again.