whenever I call ajax,
Are you trying to read a .xml, or .txt file? Or calling a .asmx web service? Check the handler mappings and see if it's enabled and if there are any request restrictions.
Check the IIS logs to see if there is any additional info about the request.
Enable failed requested tracing to get detailed error info.
https://www.bing.com/search?q=iis+failed+request+tracing