After a partial upgrade (suggested by aptitude):

[...]
Setting up texlive-fonts-recommended-doc (2026.20260527-1) ...
Setting up texlive-pstricks-doc (2026.20260527-1) ...
Setting up texlive-science-doc (2026.20260527-1) ...
Setting up libtexlua53-5:amd64 (2026.20260303.78225+ds-4) ...
Setting up texlive-fonts-extra-doc (2026.20260527-1) ...
Setting up texlive-fonts-extra-links (2026.20260527-1) ...
Setting up libkpathsea6:amd64 (2026.20260303.78225+ds-4) ...
Setting up texlive-latex-recommended-doc (2026.20260527-1) ...
Setting up texlive-humanities-doc (2026.20260527-1) ...
Setting up texlive-pictures-doc (2026.20260527-1) ...
Setting up texlive-metapost-doc (2026.20260527-1) ...
Setting up texlive-publishers-doc (2026.20260527-1) ...
Setting up libsynctex2:amd64 (2026.20260303.78225+ds-4) ...
Setting up libptexenc1:amd64 (2026.20260303.78225+ds-4) ...
Setting up texlive-binaries (2026.20260303.78225+ds-4) ...
Setting up libsynctex-dev:amd64 (2026.20260303.78225+ds-4) ...
Processing triggers for libc-bin (2.42-16) ...
Processing triggers for man-db (2.13.1-1) ...
Processing triggers for tex-common (6.20) ...
Running mktexlsr. This may take some time... done.
Running mtxrun --generate. This may take some time... done.
Building format(s) --all.
        This may take some time...
fmtutil failed. Output has been stored in
/tmp/fmtutil.wa6bvEAf.bz2
Please include this file if you report a bug.

dpkg: error processing package tex-common (--configure):
 old tex-common package postinst maintainer script subprocess failed with exit status 1
Processing triggers for install-info (7.3-2) ...
install-info: warning: no info dir entry in `/usr/share/info/maxima-index-html.lisp.gz'
Errors were encountered while processing:
 tex-common

I've attached this /tmp/fmtutil.wa6bvEAf.bz2 file.

I can see:

[...]
fmtutil [INFO]: log file copied to: /var/lib/texmf/web2c/pdftex/mllatex.log
fmtutil [INFO]: /var/lib/texmf/web2c/pdftex/mllatex.fmt installed.
fmtutil [INFO]: --- remaking hilatex with hitex
fmtutil: running `hitex -ini   -jobname=hilatex -progname=hilatex -etex -ltx hil
atex.ini' ...
This is HiTeX 2.0, TeX version 3.141592653 (TeX Live 2026) (INITEX)
entering extended mode
(/usr/share/texlive/texmf-dist/tex/hitex/base/hilatex.ini
(/usr/share/texlive/texmf-dist/tex/latex/base/latex.ltx
(/usr/share/texlive/texmf-dist/tex/latex/base/texsys.cfg)
./texsys.aux found


\@currdir set to: ./.


Assuming \openin and \input
have the same search path.


Defining UNIX/DOS style filename parser.

catcodes, registers, parameters,
LaTeX2e <2025-11-01>
(/usr/share/texlive/texmf-dist/tex/latex/l3kernel/expl3.ltx
(/usr/share/texlive/texmf-dist/tex/latex/l3kernel/expl3-code.tex
! Use of \??? doesn't match its definition.
<argument> \???
                 ! LaTeX Error: Charcode requested out of engine range.
l.35568   }

! Use of \??? doesn't match its definition.
<argument> \???
                 ! LaTeX Error: Charcode requested out of engine range.
l.35568   }


! LaTeX Error: Control sequence \c__codepoint_nfd__tl already defined.
[...]
(That makes 100 errors; please try again.)
Transcript written on hilatex.log.
fmtutil [INFO]: log file copied to: /var/lib/texmf/web2c/hitex/hilatex.log
[...]
fmtutil [INFO]: log file copied to: /var/lib/texmf/web2c/pdftex/mltex.log
fmtutil [INFO]: /var/lib/texmf/web2c/pdftex/mltex.fmt installed.
fmtutil [ERROR]: no (or empty) hilatex.fmt made by: hitex -ini   -jobname=hilatex -progname=hilatex -etex -ltx hilatex.ini </dev/null
fmtutil [INFO]: disabled formats: 3
fmtutil [INFO]: successfully rebuilt formats: 49
fmtutil [INFO]: failed to build: 1 (hitex/hilatex)
fmtutil [INFO]: total formats: 53
fmtutil [INFO]: executable location: /usr/bin/fmtutil
fmtutil [INFO]: exiting with status 1

Am 29.05.2026 um 13:41 schrieb Vincent Lefevre:

Hello Vincent,

A failing setup of tex-common is seldom an issue in tex-common itself
Until now I failed to reproduce the issue, even when calling fmtutil-sys
--all that format is not created, I'll try to have a closer look tomorrow.

Lowering to serious as in an expected installation, there are
no issues, but I don't know which is the culprit package. This
could be a missing Depends, Breaks, and/or Conflicts, or perhaps
a valid (but unusual) situation not expected by tex-common.

Note that "apt install -f" was still giving the same error.

The packages of texlive-bin source were upgraded from
  2025.20250727.75242+ds-5+b2
to
  2026.20260303.78225+ds-4
but the packages of texlive-base and of texlive-extra sources
were still at 2025.20260124-1.

Upgrading the remaining to the 2026.* versions made the error
disappear.

If the mixed 2025 + 2026 packages cannot be handled, the situation
should be disallowed via some Depends, Breaks, and/or Conflicts
declaration in particular to ensure that dependency resolution
cannot yield such a situation.

Am 30.05.2026 um 01:17 schrieb Vincent Lefevre:

Hello Vincent,
question) has been upgraded, when texlive-bin is upgraded. I don't see
partial upgrades as normal use cases, hence I lower the severity to
important.

Thanks for the test + report anyway