I just spent the last few hours troubleshooting and have determined that the problem lies with the theme "Karma" by "TrueThemes" version 1.2. When I install a basic theme like 2012, the Google Comprehensive Plugin works perfectly. With a working page in that theme, when I reinstall the "Karma" them, then the GCMP page fails again. So, it's theme-specific as far as I can tell.
When comparing this plugin between two WP 3.5.1 sites, the plug-in works on a new site with almost no other plug-ins installed, and fails on the other, more established site that has numerous plug-ins installed. Several posts report problems with the "Add" (Address) button. In my case it has no result, in another it deletes the text entered.
The site that works gets a response, with a closeable red box with the address text contained within, and the map appears normal on a new page based on a wide, open template.
The site that fails will not accept the text. I've tried URL's from Google Maps, and pasted address text directly from Google Maps. Text that works on Site #1 will not work on Site #2.
Also, the Short Code generated from Site #1 will not work when pasted into Site #2, so I assume the problem is not with the Short Code generated, but with something else.
(NOTE: I disabled all installed Plug-ins before doing the following:)
When pasting a URL (copied directly from the Address Bar of a working Google Maps location), I get the following error message in an orange pop-up:
ATTENTION! (by Comprehensive Google Map Plugin)
Dear blog/website owner,
Google returned the following error when trying to load KML file:
The KML file is not a valid KML, KMZ or GeoRSS document. (INVALID_DOCUMENT)
Re-pasting this code back into a browser results in a good map, so it's not the URL text.
Thanks all I can think of to do.