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 5 L3 programming layer <2020-08-07> (/usr/local/texlive/2020/texmf-dist/tex/latex/base/article.cls Document Class: article 2020/04/10 v1.4m Standard LaTeX document class (/usr/local/texlive/2020/texmf-dist/tex/latex/base/size12.clo)) (/usr/local/texlive/2020/texmf-dist/tex/latex/fontspec/fontspec.sty (/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/xparse/xparse.sty (/usr/local/texlive/2020/texmf-dist/tex/latex/l3kernel/expl3.sty ! LaTeX Error: Missing \begin{document}. See the LaTeX manual or LaTeX Companion for explanation. Type H <return> for immediate help. ... l.77 \ExplLoaderFileDate{expl3.sty}}
This is on a mac running Big Sur 11.2.1 with a fresh MacTeX (Texlive 2020) installation with all packages updated.
Following some advice I found online, I tried running
fmtutil-sys --all
But that didn't help either, so now I am stuck.
The computer did have previous latex installations, so it's conceivable that there is some conflict somewhere that I am not aware of and that TexLive fails to clear.