At my place after many years after replacing the router from EdgeMax to Cloud Gateways 2 weeks ago, I also had a similar problem and only added the rule I gave in another topic. https://forum.cloudron.io/post/98217
It has been working properly ever since. I don't open other ports besides http,https or add domain in DNS tab.
Why so? In the Ubiquiti system, the loopback works between the WAN and LAN, so the “LAN IN” rule activates the loopback correctly. Because the loopback “works” in the address network like Gateway, so, then when I added blocking access to Gateway from different VLANs(probably at this point, because I configured in stages throughout the week), then I could not turn on the site on the computer through the local network, and outside with no problem. I started snooping what was the cause, reviewed the rules in the firewall and the forum. I read somewhere that you had to add a rule in LAN IN. After adding it started working as it should, even the tests in Wordpress shows correctly.