I have a test install of Moodle 2.01+using the Base theme.

I've just started creating a test set up but have come up with the following error whilst creating categories.

The attached error message only shows in Internet Explorer and won't let me progress to a course that I've created under the last category of the screen shot - works OK in Firefox, Chrome and Opera.

My initial test site install was using Moodle 2.0 when I came across this issue. Only changes I'd made was to the Theme "Splash" by adding the path to a graphic for a logo plus added some entries for the top drop down menu.
I then upgraded to 2.01+ and still had the error as detailed.
What I did notice after the upgrade was that the theme Spash seemed to have lost it's formatting even after a cache clear.
So I changed to Base theme to explore from there.
Interestingly I was able to navigate to my test course using IE 8 via the Navigation menu in the side bar OK
Rob