I am having the same issue, which is very aggravating. I have apparently learned 50 new words for the day & got locked out of the flashcards.
According to the free v. paid subscription details, we should be allowed to continue reviewing as free users ("Limited to 50 new words per day, but review as many as you need").
Either this misleading statement should be taken out, or the bug that prevents us from reviewing old words should be fixed.
I hope someone from lingvist sees @Justin-Huang 's post soon and corrects the error. The app really loses its usefulness & appeal if I'm not allowed to even practice Spanish. . .