Unable to create new menu item

I installed this (superb!!) module on my testbed (Windows workstation), using Firefox 2.0.0.6 (English version). It worked great! So now I have installed it on my development site and started to use it. Everything seems to work fine... until I try to add a new menu item. At this point, nothing happens at all. When I look at Firebug, I see that pressing New menu item causes this message to appear (in red):

Here is the configuration:
JS Query 1.1.2
Firefox 2.0.0.6 (French version)
Drupal 5.1
Interface Library and Update 5.x-1.0
QAM: 5.x-1.0-beta

I have installed the new JSQuery (I believe - at least the version number that shows is the same on the site that works and the site that doesn't).

PHP is version 5.2.3 in both cases

Ironically, it works fine in IE7, except that when you try to update the modified menu everything goes belly-up because the XML is not recognised.

I have not installed any of the patches in the support forum, because they seem to be about different problems which I am not experiencing. The only thing that I can think of that is different in my browser is that the version that works is the English version and the one that doesn't is French - but does this make a difference?

Help most gratefully received since this module will save my life if it works...