Website rejecting all inbound traffic after moving nameservers to Cloudflare

I recently moved my website’s nameservers to Cloudflare, and now I’m facing a major issue—the site is rejecting all inbound traffic! I noticed a similar issue was posted here before, but I haven’t found a clear solution yet.

Here’s what’s happening:

:small_blue_diamond: Error Messages: Some visitors report getting “403 Forbidden” or “Error 521: Web Server is Down.”
:small_blue_diamond: Cloudflare Settings: I checked my DNS settings, and everything seems correct. But could it be a misconfigured SSL/TLS setting?
:small_blue_diamond: Firewall & Security Rules: I didn’t set any strict firewall rules, but could Cloudflare’s security settings be blocking legitimate users?
:small_blue_diamond: Server-Side Issues: Could this be an issue with my hosting provider rejecting Cloudflare’s IPs? If so, how do I allow them?

Maybe I’m missing something in my setup?

Has anyone dealt with this before? What’s the best way to troubleshoot and fix this issue?