so I came across this strange "bug" or whatever it is about a handful of times in the last maybe two weeks. out of nowhere the new terms in some vocab schedules can't be introduced as new terms. they are marked red (= „You have selected only kanji-related questions, and some words a)do not have kanji or b)do not have kanji that you know.“). Both of these reasons aren't the case though.
So far my only fix was to copy the schedule. the copy immediately works fine. right now the problem occurred in my schedule "何".
no, seems like it also solves itself after a while. (so far I always did the copy thing immediately). I will post here if it happens again and maybe you can catch it next time before it disappears. (but it definitely was there for several hours and also after the reset due to the next day I think).