Page not found error when using panels
Turned on logging and saw Rejected-By-UrlScan next to the 404 errors.

Since our site is hosted in IIS, I needed to go into URLScan config, and allow colons, which are used by the Panels ajax func

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

; :   ; Don't allow alternate stream access