I've run into an odd error.
My account is set to ask review questions on lesson pages, drawn from lesson vocabulary. As of today, the window loads ("Before we continue, five questions to review....") but the questions do not. I get that spinning wheel, forever. This happens on all lesson pages.
Some other points: in the Study Center I can still review as normal. I can't review from the Vocabulary tab.
It seems that I can study forgotten words as they come up for review, but words from my vocab list are not loading.