I was experiencing the folloiwing problem: i am logged in and in the admin panel and everytime when i click on "Proxy Manager" i was getting ErrorDocument 406 with the following message:
An appropriate representation of the requested resource /proxy.php could not be found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request."
After some research, testing and excluding different possibilities - i have found what is the problem. It turns out that some shared hosting providers block access to certain files with specific names like "proxy.php". So if i rename the file "proxy.php" to something that not contain the word "proxy" - then everything is fine. So i have checked all the files and i have replaced all appereances of "proxy.php" with "privacy.php" - 12 files with 25 appereances - and now everything works fine Note that for example "spproxy.php" or "seoproxy.php" was triggering the same error and ErrorDocument 406.
I hope this info will help you!
Thanks to Angelov for finding the issue.