Occasional 504 error, most definitely no issues at our end

Nobody else has responded, so I’ll give it a try.

Are you getting the pretty Cloudflare error screen? The Support docs say it’s getting the 504 from your server, but you say there are no error messages. I know you said you have two domains configured the same way and one is ok.

Could there be a firewall that’s triggering and blocking access?

Is this on the busier of the two domains?

https://support.cloudflare.com/hc/en-us/articles/218378978-What-should-I-do-after-seeing-a-502-or-504-gateway-error-on-my-site-