This is an error that can be caused by quite a few things; In my experience, this
error occurs mot commonly when the server has been set up with firewall rules and/or other security
features such as mod_security,, etc.
Attempt to get in tocuh with your host and ask them to provide the error logs. This
should point you in the right direction. They may even be able to assist with removing the
security features that might be causing the issue.