I can’t start any new vocab lesson today, either by clicking on the global button on the dashboard or by clicking 勉強する on the deck itself:
As you can see I still have nearly 200 unstudied items in that N5 deck:
I tried switching the メインデッキ to Vocab N4 and I could do new lessons again, then switched back to N5 and I get the same 学習済です message. Looks like it won’t let me study from N5.
If I go to a yet-unstudied N5 entry I see nothing out of the ordinary:
If I click the 「練習に追加する+」 button it does get added to my reviews.