Error: /webwork2/instructorXMLHandler: Forbidden
(1) The most frequent cause of this is that the $site_server_url value in file webwork2/conf/site.conf is set incorrectly. For example http://mysite.edu is used instead of https://mysite.edu or vice-versa. Browsers can self-correct the url if they get an error message on the first try but the AJAX calls simply fail so it is important that the $site_server_url be exactly correct.
(2) A recent occurrence of this error was due to the fact that in the Apache httpd.conf file the requests addressed to https://mysite.edu were automatically redirected to https://mysite.edu/webwork2 (presumably to make the url easier to remember and/or type). In this case way the redirection command was written meant that calls to https://mysite.edu/mod_xmlrpc (the address for AJAX calls) was redirected to https://mysite.edu/webwork2/mod_xmlrpc and that caused the instructorXML error. It took a while to track down.
There are other issues that can cause the AJAX call to fail, some of them remain a mystery.
See the threads: