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When I try to compile some TeX code from a few years ago that relies on the siunitx package, I get the following error:

! 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 2021-02-18, but in your TeX tree the files require
(LaTeX3)        at least 2021-05-11.

For immediate help type H <return>.
 ...                                              
                                                  
l.77      \ExplLoaderFileDate{expl3.sty}}
                                         %

The code itself is not relevant; the error can be reproduced by compiling, say, the following MWE:

\documentclass{article}

\usepackage{siunitx}

\begin{document}

\SI{1}{\s}

\end{document}

I have just updated my TeX Live 2021 distribution and furthermore I haven't encountered this error before.

How do I fix this?

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    What is the output of kpsewhich --all --engine=pdftex pdflatex.fmt? (assuming you are using pdfTeX) Commented May 26, 2021 at 0:29
  • [<username>@localhost test]$ kpsewhich --all --engine=pdftex pdflatex.fmt /home/<username>/.texlive2021/texmf-var/web2c/pdftex/pdflatex.fmt /usr/local/texlive/2021/texmf-var/web2c/pdftex/pdflatex.fmt Commented May 26, 2021 at 0:37
  • 1
    Delete /home/<username>/.texlive2021/texmf-var/web2c/pdftex/pdflatex.fmt and it should work Commented May 26, 2021 at 0:37
  • Thank you very much! Commented May 26, 2021 at 0:39
  • This is in theory a duplicate of tex.stackexchange.com/q/585664/134574, but that post doesn't have an answer... Commented May 26, 2021 at 0:45

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