File `l3backend-pdfmode.def' not found (TeX Live 2020)

Question

I just finished installing and updating TeX Live 2020 on Windows 10. I performed a full installation of TeX Live. Now, compiling this simple document:

% !TEX program = lualatex
\documentclass[a4paper, 10pt, extrafontsizes]{memoir}

\begin{document}
Text
\end{document}

I receive this error message:

This is LuaHBTeX, Version 1.12.0 (TeX Live 2020/W32TeX) 
 restricted system commands enabled.
(c:/Users/marco/Downloads/temp/temp.tex
LaTeX2e <2020-02-02> patch level 5
 L3 programming layer <2020-03-06>
Calling deprecated Lua function strcmp (c:/texlive/2020/texmf-dist/tex/latex/memoir/memoir.cls
Document Class: memoir 2020/10/04 v3.7n configurable book, report, article document class
(c:/texlive/2020/texmf-dist/tex/generic/iftex/iftex.sty) (c:/texlive/2020/texmf-dist/tex/latex/lm/t1lmr.fd) (c:/texlive/2020/texmf-dist/tex/latex/memoir/mem10.clo) (c:/texlive/2020/texmf-dist/tex/latex/lm/lmodern.sty) (c:/texlive/2020/texmf-dist/tex/latex/base/fontenc.sty) (c:/texlive/2020/texmf-dist/tex/latex/tools/array.sty) (c:/texlive/2020/texmf-dist/tex/latex/tools/dcolumn.sty) (c:/texlive/2020/texmf-dist/tex/latex/tools/delarray.sty) (c:/texlive/2020/texmf-dist/tex/latex/tools/tabularx.sty) (c:/texlive/2020/texmf-dist/tex/latex/textcase/textcase.sty))

! LaTeX Error: File `l3backend-pdfmode.def' not found.

How can I solve this problem? Thank you very much for your help!

Answer 1

I experienced the same issue after a recent tlmgr update --all with texlive 2020. Reinstalling l3backends, l3kernel didn't help.

The changelog for l3backends reads:

[2020-09-01]

Changed

...

• Split pdfmode driver into pdfTeX- and LuaTeX-specific files ...

so, evidently my recent update picked up this change (it's been a while since I updated).

Then I noticed that when running pdflatex I get:

LaTeX2e <2020-02-02> patch level 5 L3 programming layer <2020-04-06>

So an old L3 version (which preceds the change to l3backend change) is being used, even though L3 had supposedly just been updated to something more recent.

Just as an extra data point, I also noticed that xetex didn't seem to have similar issues.

Following the comment by @David_Carlisle, I ran kpsewhich --all -engine=pdftex pdflatex.fmt and found that I have two locations reported on my system:

/home/craig/.texlive2020/texmf-var/web2c/pdftex/pdflatex.fmt /home/craig/texlive/2020/texmf-var/web2c/pdftex/pdflatex.fmt

I renamed the first one, and the problem went away.

Some related issues suggested that you may need to run mktexlsr, though I didn't need to.

Good luck.