Well... it's working now. It's mysterious because I didn't do anything at all. I submitted a support ticket and all they said to do was to clear cookies and cache, and I didn't get around to doing that yet.... and just checked it now "just in case" - and all is well. So, since I didn't clear anything, and yet it's working again, cookies/cache were not the cause of the problem.
I'm glad they fixed it!