I've seen that message too in the past few weeks, and I still get that message when I use Shift-reload (a hard reload) in Firefox. When I do the same in Chromium, everything works fine.
My guess is that a hard reload doesn't cache anything, and somehow Cloudflare doesn't like that. Could it be you've disabled all browser cache?
I'm using Firefox too, but hard refresh makes no difference to me, though you may be on to something since about:cache reveals I'm apparently not caching
anything from the forum. I definitely
used to since I remember avatar updates not showing up immediately except with a hard refresh or otherwise clearing the cache. Other sites are caching normally, and I coincidentally haven't (recently) had any weird Cloudflare issues with other sites, only this forum. There's definitely something going on here.