1. Use IE7
2. Create a web content structure with a repeatable HTML field
3. Start creating a basic web content, and change the structure to the one you created, using the structure selector at the top of the form
4. At the top right corner of the HTML editor field, click on the "+" icon in order to add a new field
The reason is that the Ajax request being sent for rendering the editor is too long (~3000 bytes or even more). You can examine this with a HTTP request monitor application e.g. Fiddler. The long URL in the request is due to many unnecessary request parameters that are not actually needed to render the editor.
Trunk is not affected, but WC structures are completely refactored to use DDM, so this has to be fixed on 6.1.x.