There are a number of cryptic server errors that are displayed that don't give you any information on what is causing the error.
One of the more common errors is the 500 server error and can be caused by a number of things. The easiest way to get more information is to log into your cPanel and view the Error Log in the Logs section. Common causes are invalid htaccess entries, permissions set to 777 instead of 755, and premature end of script headers which is a problem with your script.
A 406 error is most likely caused by mod_security which helps protect against a number of hacking attempts. If you think you are getting a 406 in error, open a support ticket and give us the URL that's causing the error and we'll modify the mod_security rules so your URL works properly.