amber nest
Would you mind sharing the domain, as well as some of the domains that use the same IP but are working?
Do you have more specific times as to when the issue started/stopped? Especially, do you know the time the issue stopped the first time?
When you say "I have Zero Trust installed", what exactly do you mean? Access?
amber nest
From the timestamps, it doesn't seem to be related to the other problem that I had in mind.
The domain that isn't working has different Edge IPs for me from the domains that are working. Are they in the same account/ same plan level(free/paid)?
Can you send some example requests to the domain that doesn't work, but use the IP of one of the domains that does work? As the domain is behind Access, I can't test myself.
amber nest
Can you just check that you're using the Full (Strict) SSL mode on Cloudflare? Can you confirm the domain is working (from an external source) if you bypass Cloudflare and connect to the IP address directly? A 523 on one domain is just really strange if a different domain works with the exact same A record.
The only thing that comes to my mind is an actual problem with the Zero Trust IP address, that maybe it is not allowed to reach your server.
Can you try a traceroute to the last IPs that Cloudflare used to connect to the domain in question?