LaTeX.org

O.k. Thanks! That explains the immediate problem though I cannot figure out the underlying cause.

After your comment, I checked pdflatex, and with pdflatex I do get the most up-to-date versions:


LaTeX2e <2020-10-01> patch level 4
L3 programming layer <2021-02-06>


and

This is pdfTeX, Version ...

Regular latex works fine, but xelatex on any document with fontspec fails with an error such as the following:
This is XeTeX, Version 3.14159265-2.6-0.999992 (TeX Live 2020) (preloaded format=xelatex)
restricted \write18 enabled.
entering extended mode
(./test.tex
LaTeX2e <2020-02-02> patch level ...