The solution should be なんとう / ナントウ, at least accroding to the lesson.
I am using the userscript “WaniKani Katakana for On’yomi”, which is why the input at the bottom is in katakana. I have not had problems with this userscript yet, so I am unsure if it is causing a problem here.
Am I doing something wrong? What exactly causes the error here?
It’s best to ask these questions on the extensions’ threads, since these might not be seen, especially with a vague title like that.
Btw, just disable all your scripts, test if it works now, then enable them one by one, to see which one breaks it. That will be the one you need to ask questions for.
As far as I’m aware, the script katakana for onyomi is only for kanji. Is/was your caps lock on? Because that will convert hiragana to katakana and I’m pretty sure wanikani doesn’t like it when you answer vocab readings with katakana.
You typed in katakana and since some words are katakana, it wants hirigana. Don’t use your IME when doing your reviews or the in-lesson/end-of-lesson reviews.
But, to answer the question in your topic title: yes some userscripts can and do break lessons and/or reviews. If something seems off, rule of thumb is to try turning off all your userscripts and see if it happens again or not, before reporting a bug. Or, if you only have 1 userscript and you’re sure that’s the cause, go to that script’s thread and report it to the dev there. However, this only works if it’s for a newer script that is still being maintained…