Error 521 - Can’t figure out why - Works when accessing the IP directly and through other cloudflare domains

I’m trying to host a webserver using IIS on my server running Windows Server 2019.
The website is forwarded through a linux firewall from the webservers local IP to my public IP.
I can access my website when going directly to my local and public IP, but not through Cloudflare ssl.
I’ve checked the access logs for IIS and there is no mention of any activity from Cloudflare.
I have two other Cloudflare domains which are hosted on the same server and through the same firewall, and they work perfectly.
The site I have issues with works fine when I route it through one of my other domains.
Cloudflares ip adresses are whitelisted with iptables on the linux firewall.

My issue is very similar to the one I had here, which magically fixed itself.

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