Saving document to WSS gives script error

Question

I have a custom content type with two extra columns defined in WSS 3.0. I have a document list with word documents of that content type.

When I open a document on my XP client with MS Office 2003 and IE6 (company regulations :-( ) this works fine. When I make changes however and I try to save the document, I get following error:

I don't know what is causing this: IE6, Word 2003, WSS 3.0 ... Does anyone have an idea?

Thanks, Jelle