For me this bug seems to be related for unraid webgui tab being open too long (Chrome). Normally I have my computer open 24/7 and also unraid webgui tab open at browser. This haven't been issue until upgrading to 6.12.2. After this upgrade I experienced webgui crash twice. On my server it seems not to be related to IPv6 because I've always had IPv4 only setting.
I couldn't get webgui back although I killed nginx and tried to restart and reload it and also tried to restart and reload php but it didn't make any difference. Otherwise everything worked normally, VMs and dockers kept running and smb shares worked and I could connect to server by ssh. I could get to unraid login screen if I cleared cookies but after login I just got http error 500. Only way to get webgui back was to restart whole server.
Only common log entry before both crashes was this, it was repeated at 1 hour interval until I noticed webgui crash and rebooted:
I don't know if that is cause or just effect after webgui has been crashed or is it any way related to issue but that was on log before both crashes and I haven't seen that after those crashes.
Now I've kept unraid webgui tab closed at browser when not operating and webgui has been up +24 hours. Let's see if it's just good luck.