Well, I spoke too soon. After disabling Cloudflare:
I still cannot react,
I cannot recommend a post
can create new threads
when I reply, I sometimes get the whole page to appear in the field, and a sign-in button (as if I am not signed in already) and once I do re-sign in, I can post
I cleared the system cache, I cleared it in my WP installation too (in case it could affect it). I even tried with a different computer. There must still be something wrong.