I would like to add another data point for this issue.
I'm new to the unraid and just set it up for 2 weeks. I started with 6.12.2. Everthing works great until 2days ago.
The symptom is exactly same as this post. The webgui crashed after a few hours running and I have to kill the nginx and restart it to access the webgui. More interesting thing is that, I restarted my server when the first time I occured this issue. At that time, even though the array haven't running, all the dockers and VMs haven't started, the webgui was still crashed.
After reading though this post, I highly suspect this is related to ipv6. Because as far as I remember, the last thing I was doing on my server is setting up Qbittorrnet docker. One process I'm done is enable ipv6 in my unraid network setting since the bt tracker I'm using only allow ipv6 connection.
Let me know if I should attach anything here or make a new post to help solve this issue. Thanks!