I just tried turning customizations back on and that doesn't seem to make a difference.
I'm a computer programmer, so debugging is my life.
My next theory: when I try to use the "quick reply" dialog at the bottom of the blog, my post fails. HOWEVER, when I post with the full reply form that includes a title, my post succeeds. Going Advanced takes me to this form, and so does the error screen for quick posting. I wonder if it's a problem with the "quick reply" form.