Javascript error: Error in Drupal.nrembrowser.nrembrowserPassSelection (tabs list): error

First off, thanks for this module, it does exactly what I need.

I'm experiencing some problems with this module on some of my sites (others work perfectly). I've been debugging what I can, but I'm out of my depth here, and I'm hoping that someone might have some suggestions on what I might be able to look at next to try to figure this out. Here's some background:

Running a multisite environment. In this environment I'm sharing all module code for all modules as well as the theme so I can confirm that the code for the sites that are working is the same as the sites that aren't working. Modules installed (all 6.x):
- nrembrowser 1.0-beta 15 (also tried beta 14)
- CCK 2.9
- jquery ui 1.4
- jquery_update 2.0-alpha 1
- Wysiwyg 2.2 (tried with CKEditor, FCKEditor, TinyMCE)
- Views 2.12
- Vertical tabs 1.0-rc1

When I click on the nrembrowser button in the Wysiwyg toolbar (or add content link for node reference field) the popup screen displays, all of the tabs appear and the little blue spinner appears beside the all tab. Then the javascript alert box appears. When I click OK on the alert box I get a white screen.

The error message is coming from the nrembrowser.widget.js file on line 256. I've attached screenshots of the error message as well as what I can see in the XMLHttpRequest variable in firebug.

I have also done some checking on the php side and have determined that the function nrembrowser_add_media_page function runs to completion, but it does not get to the function nrembrowser_media_tab_page before the error is thrown, so it's failing somewhere between these two, but I haven't been able to nail down where.

I've also been comparing modules installed, themes, source code from the edit page, etc between the sites that work and those that don't to see if I can find any significant differences that might be causing the problem.

I'm stumped , any suggestions as to where to look next? What should run next after the nrembrowser_add_media_page function?

Thanks