I am running into similar issues after upgrading to 6.12.4. All plugins removed or updated. As soon as the array is started in Maintenance Mode, Normally, with docker on and off, etc. The only way I am able to access the webGUI after array start is within safe mode without plugins. I thought I'd held off long enough to upgrade to avoid breaking issues, but I guess not.
I was able to get it working initially. I installed the new appdata.backup plugin. Halfway through the first backup is when issues began. Docker runs in safe mode. It is something in the plugins. I already attempted altering the SSL port suggested above. Can provide full syslogs if needed. Right now managing to get by without plugins but I can't stay this way. Shortly after Party tips & tweaks starts and you see remote shares mounting is the point where the WebGUI dies.
servers.conf ungootmedia-diagnostics-20231025-1507.zip ungootmedia-diagnostics-20231025-1510.zip