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.
EDIT: Randomly poking at cache settings seems to have fixed the caching issues, as well as causing the error to be reliably triggered by hard refresh. But only on this site, though: other Cloudflare sites don't take issue with hard refreshing, or even with completely disabling caching. It seems Cloudflare expecting a functional browser cache, but only on this forum, combined with some kind of weird browser cache corruption on my end, was the source of the trouble. The weirdest part (one of the weirdest parts, anyway) is that I cleared my cache several times during all of this, which didn't fix it, and I still don't know what
did fix it.