In the Tae Kim deck, some cards on the question side are omitting parts.
It shows up as: 先生が学校に......notか知らない。
but when I click "edit current" it still says: 先生が学校に行[...]か行[...not]か知らない。
Can anyone tell me what's going on? :/

Those are cloze deleted cards.

That´s how they're supposed to be. That deck is for learning grammar. It´s "fill-in-the-blank," not "read this." If you´re just going to be reading, you might as well be reading something more interesting than random prepackaged sentences.

The deck also uses that little box where you can type in the missing stuff to check your answer.

Oh yeah I know, but it's supposed to show the 行[...] and 行[...not], and it's suddenly not showing it anymore.

It's not showing the 行 kanji or the [] brackets, and I kinda need those, at least the kanji.

Seems there's something weird going on with the Japanese support plugin, I can disable and restart and it shows up fine. Hmm. :/

The cloze looks like furigana. Edit the plugin and disable furigana on arbitrary fields to fix the problem, and please report Anki issues on the Anki forum in the future.