Safari fails after Monterey upgrade with "(NSPOSIXErrorDomain:28)"

brbo,

I appreciate your effort to capture the obvious things first. That's always good debugging practice. Let me address your suggestions:

> Seems Safari needs more free space than your Mac now has free, after the update.

Given the message, this was an obvious possibility that I had already checked: I have multiple drives attached to the Mac mini, but all of them have ample space -- the lowest being 266Gig free. I also did a "df -h" on the machine and compared it to another Mac that was still running the previous OS, and saw no obvious capacity issues.

> If so, with several tabs open Safari.could be struggling.

I don’t typically keep a lot of tabs open, but I can verify that I can recreate the issue with only a single tab in a single window, just by browsing through various websites for some period of time (less than 5 minutes). 

It seems like there *might* be a correlation with pages that have lots of embedded YouTube videos, but I have no solid proof of that.

> Activity monitor on your Mac is a handy way to see which apps are using all the energy:

I am familiar with Activity Monitor and use it regularly. There was nothing unusual on any of the tabs.

As to the “If Safari on Mac doesn't open a webpage or isn’t working as expected:” article:

• Reload the page — reloading does nothing; once Safari is in this state opening any page shows the error
• Install software updates — Already running latest: OS 12.0.1 + Safari 15.1
• Turn off Safari extensions — No affect after disabling and removing all extensions other than 1Password. I will disable this as well for testing, but a web browser without access to the password safe is unusable.
• Test with a private window — I was able to replicate even after clearing all history, but I will test in a private window today.
• Check network settings — no proxy or custom DNS settings on this machine. Other applications (including Chrome looking at the same pages being tested in Safari) all continue to work.

Anyway, I hope you see that I had done some fairly extensive testing before posting the issue here. Apologies for not providing the above detail in my original post.