Thank you very much for the great support you guys provide, that was the problem and namecheap’s support fixed in via “support chat” in less than 5 mins
here is the issue in case someone finds this thread in the future with the same problem
from Namecheap’s Support
“Well, that’s because of Apache’s web application firewall, mod_security.
It seems like the issue was caused by the fact that some of your site’s configuration have triggered a mod_security rule. I’ve whitelisted it, so now it should be working properly. Please try it out now.
It blocks known exploits and provides protection from a range of attacks against web applications. Sometimes, ModSecurity rule can be invoked despite the validity of the request.
In such case, you’ll need to contact us to get it whitelisted.”