I just updated my TeXLive installation on my iMac and got the following error message:
/usr/local/texlive/2020/texmf-dist/tex/latex/l3kernel/expl3.sty:77: LaTeX3 Error: Mismatched LaTeX support files detected. (LaTeX3) Loading 'expl3.sty' aborted! (LaTeX3) (LaTeX3) The L3 programming layer in the LaTeX format (LaTeX3) is dated 2020-10-27, but in your TeX tree the files require (LaTeX3) at least 2020-12-07.
What to do?