I have problem with accessing sites protected by CloudFlare. First time I visit, it checks if I’m human and works fine. When I open the page second time (and any other time), CloudFlare checks again, but gets stuck in the loop and I cannot get to the site. Same in anonymous and normal mode.
I’d also try testing with browser extensions temporarily disabled and clearing the site cookies for the affected page. Cloudflare verification can be surprisingly sensitive to cached data or privacy settings, so those simple checks sometimes solve it.
Have made a separate thread detailing a root cause, which is also likely what happened here. No way to know for sure, but given the OP’s post relates to a forum, it was likely running XenForo (the same forum software as my situation - not the cause, but it uses a service worker).