It sounds very much like a mod_security related problem. See http://wiki.modxcms.com/index.php/What_is_mod_security_and_how_does_it_affect_me
I have followed all the instructions to add the WebLoginPE snippet to a site. However when I copy and paste the php code to the snippet and save, I get a message saying that it is saving but then it doesn’t actually save. I created an empty snippet first and saved it, then went back and added the php and failed to save. I don’t even get any error messages. IS this anything to do with mod security?