Next button breaks during review sessions

I’ve been getting this issue for the past day or two. Each time I do a review session, sooner or later it stops moving to the next word. The enter key, the “next” button and the “see answer” button stop working so I have to save and exit or press the cross button to get out of it.

I think I’ve found the cause as this appears in the console each time it breaks:


It seems to happen at a random time during review sessions. Sometimes it happens near the start, sometimes it happens near the end, but I’m getting it time after time and it’s annoying.

I’m using the web version of Memrise through Firefox.

Edit: I’ve been having this problem for over a week now and only a handful of people have looked at this thread so I guess few if any others are experiencing this issue… If anyone can give any insight as to what’s causing it, if there’s anything I can do on my end to fix it or post if they’re also getting this issue so it’s known that I’m not the only one then that’d be appreciated.