Creating a new topic under General to see what happens.
Seems to have worked OK. “Blog” was selected by default in the category box, though—that might be the problem. Ideally, the category in the category box when you create a new topic should reflect the currently selected category, if you’re in one.
Edit - Looks like it does. However, “Blog” is the default category in the box when you’re not actually viewing a category.
The first time, I accidentally created it using the default category, but the second time, I chose General. Odd. Maybe try with a non-admin account?
Well, it worked that time. No idea what happened earlier.