Unable to access HA web interface, but HA is definitely still running because HomeKit bridge still works

Hello everyone. First of all I would like to apologize if I posted this in the wrong category. If this is not the correct category, please kindly point me to the correct category.

So today I am suddenly unable to access the interface from the web or the mobile app. But I know Home Assistant is definitely running because the HomeKit bridge is still working, so I can still control appliances using Apple’s Home App.

I restarted the server several times but I still can’t access the interface.I usually access the interface through the URL https://myurl.duckdns.org:8123 because I have configured DuckDNS for remote access as well. It could only be accessed if the home network is connected to the internet, even when it’s accessing from the server machine itself. But the server machine is definitely connected to the internet as I am able to access google.com through the web browser.

The server machine is an old Asus Eee PC laptop connected to the home network with an ethernet cable. I am running Home Assistant Supervised (Docker container) on Ubuntu Desktop 20.04. When I open Portainer, all the containers related to Home Assistant all seem to be running. Could anyone please help me troubleshoot because I have no idea where to begin. I really don’t want to have to do a fresh install just to get it working again. Any suggestions would be highly appreciated. Thanks!