As matmarex suggested, I tried to login at https://csb.wiktionary.org/ and... it worked! When I navigated back to English Wikipedia, I had to login again but this time it was successful (though I have no idea why). I have now completed a few successful edits on Wikipedia, logged out, then logged in again. The problem seems to be fixed, so thanks matmarex!

More info: the actual login error message I receive is on a Special page that says