From what I can find regarding that error, it sounds like Safari is the problem. Basically, userscripts and other plugins should be excluded from Content Security Policy, but Safari isn’t doing that and other browsers are.
The strange part, though, is that I haven’t seen any other Safari users mention this, and it also sounds like scripts were working for you until recently. That makes me wonder if maybe Wanikani changed their Content Security Policy recently, causing this issue to appear. Or maybe this really isn’t the issue, though I haven’t found anything else that might be.
Which version of Safari are you on? After a quick look-see at the thread I’ve linked previously it appears that another user had a similar problem on Safari 9 but then updated to Safari 11 which seems to have fixed the problem. Maybe that’ll help?
For what it’s worth, I use Safari for (almost) everything else, but now use Firefox for WaniKani. My reorder script just stopped working (in Firefox) but I restarted the browser and now it’s back in business.