Panels -> Add content in panels gives AJAX HTTP-error
This worked for me: https://drupal.org/node/1036202

Posted by Kye on February 11, 2013 at 11:45am
I had the same problem today. Went through this and similar threads. We were hosting the site in IIS and the problem turned out to be URLScan, and colon in the URL plan. I had to do is exclude the colon rule and everything worked as expected.

C:\Windows\system32\inetsrv\urlscan\UrlScan.ini

; : ; Don't allow alternate stream access