We see the following Java script error with a new installation. Moodle 2.9+, windows server 2012, and iis. The error says YUI is undefined. No errors when it is run on the host computer.
YUI.add('moodle-core-io',function(Y){Y.on('io:start',function(id){M.util.js_pending('io:'+id)});Y.on('io:end',function(id){M.util.js_complete('io:'+id)})},'@VERSION@',{condition:{trigger:'io-base',when:'after'}});YUI Undefined