Something that worked this time, who knows why. I stopped the squid service and then started it using
squid -d 9
to get it to log what was going on to the console. In a separate console I ran
squid -k rotate
and then it worked without error. No idea why that would make any difference.
Found same thing was occurring on a second pfsense box, same version, same installed add-ons, much less traffic. Did the commands listed above and rotate finally worked again. I'm also killing the squid -d 9 process that I started for doing the log rotation and starting squid again through the web console.