I think pax(1) may be mis-attributing errors to Ustar when using other formats. I've been seeing this kind of error when using the pax format:
"pax: Ustar cannot archive a socket /home/example/.cache/at-spi/bus_0" Otherwise pax seems to be working okay. The file(1) command reports the archive as being pax format. I can then extract files from the archive so I'm not concerned with its operation. I noticed it just before OpenBSD 7.6 was released, so I've observed the behavior with both 7.5 and 7.6.
