This seems similar to https://redmine.pfsense.org/issues/11733, which was closed for no reason (the issue was with ONT, not LAN link, so there was no reason fro web UI to stop responding).
This apparently affects all interfaces regardless of which gateway goes down.
Right now I have WAN as Tier 1 and WAN2 as Tier 2. When WAN2 (second ISP) has packet loss, I both lose Internet connectivity using WAN and web UI becomes unresponsive.
Not always, but often, which is especially annoying during video calls.
Here are the logs from the last time it happened:
Spoiler
Dec 3 07:35:44 nginx 2022/12/03 07:35:44 [crit] 39955#100173: *31629 SSL_write() failed (13: Permission denied) while processing HTTP/2 connection, client: 192.168.1.2, server: 0.0.0.0:443
Dec 3 07:35:11 php-fpm 30078 1.0.0.1|redacted|WAN2_DHCP|2.429ms|0.833ms|0.0%|online|none
Dec 3 07:35:11 php-fpm 30078 /rc.openvpn: MONITOR: WAN2_DHCP is available now, adding to routing group MultiWAN
Dec 3 07:35:10 check_reload_status 381 Reloading filter
Dec 3 07:35:10 check_reload_status 381 Restarting OpenVPN tunnels/interfaces
Dec 3 07:35:10 check_reload_status 381 Restarting IPsec tunnels
Dec 3 07:35:10 check_reload_status 381 updating dyndns WAN2_DHCP
Dec 3 07:35:10 rc.gateway_alarm 44475 >>> Gateway alarm: WAN2_DHCP (Addr:1.0.0.1 Alarm:0 RTT:2.444ms RTTsd:.829ms Loss:0%)
Dec 3 07:35:00 sshguard 42588 Now monitoring attacks.
Dec 3 07:35:00 sshguard 48246 Exiting on signal.
Dec 3 07:34:34 php-fpm 30078 1.0.0.1|redacted|WAN2_DHCP|2.533ms|0.65ms|13%|down|highloss
Dec 3 07:34:34 php-fpm 30078 /rc.openvpn: MONITOR: WAN2_DHCP has packet loss, omitting from routing group MultiWAN
Dec 3 07:34:34 check_reload_status 381 Reloading filter
Dec 3 07:34:34 php-fpm 62018 /rc.newwanip: rc.newwanip: on (IP address: redacted) (interface: WAN2[opt1]) (real interface: vtnet1).
Dec 3 07:34:34 php-fpm 62018 /rc.newwanip: rc.newwanip: Info: starting on vtnet1.
Dec 3 07:34:33 check_reload_status 381 Reloading filter
Dec 3 07:34:33 check_reload_status 381 Restarting OpenVPN tunnels/interfaces
Dec 3 07:34:33 check_reload_status 381 Restarting IPsec tunnels
Dec 3 07:34:33 check_reload_status 381 updating dyndns WAN2_DHCP
Dec 3 07:34:33 rc.gateway_alarm 41178 >>> Gateway alarm: WAN2_DHCP (Addr:1.0.0.1 Alarm:1 RTT:2.530ms RTTsd:.653ms Loss:11%)
Dec 3 07:34:33 check_reload_status 381 rc.newwanip starting vtnet1
I didn't have this issue before Multi-WAN. Nginx error is especially concerning. That was me trying to refresh frozen page, but I was unable to do so.