Undefined control sequence \current at color with texlive 2020

[Detlev Droege]

Hi experts,

one of our internal document classes suddenly throws an
"Undefined control sequence" error after upgrading to TeXlive 2020.

I could boil it down to this minimal input:
-----------------
\documentclass{dinbrief}
\usepackage{xcolor}

\begin{document}
  Some text.
\end{document}
-----------------

running this with TeXlive 2020 latex/pdflatex/lualatex produces
-----------------
$ pdflatex minifail.tex
This is pdfTeX, Version 3.14159265-2.6-1.40.21 (TeX Live 2020)
(preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./minifail.tex
LaTeX2e <2020-10-01> patch level 3
L3 programming layer <2020-12-07> xparse <2020-03-03>
(/usr/local/texlive/2020/texmf-dist/tex/latex/dinbrief/dinbrief.cls

Document Class/Style 'dinbrief' - Version 1.73
University of Karlsruhe - 2000/03/02 12:49:21

*** dinbrief: Running in LaTeX 2e mode!
*** dinbrief: NFSS 2!
Document Class: dinbrief 2000/03/02 LaTeX2e class
(/usr/local/texlive/2020/texmf-dist/tex/latex/dinbrief/dinbrief.cfg))
(/usr/local/texlive/2020/texmf-dist/tex/latex/xcolor/xcolor.sty
(/usr/local/texlive/2020/texmf-dist/tex/latex/graphics-cfg/color.cfg)
(/usr/local/texlive/2020/texmf-dist/tex/latex/graphics-def/pdftex.def))
(./minifail.aux)
! Undefined control sequence.
\set at color ...\@pdfcolorstack push{\current at color
                                                  }\aftergroup \reset at color
\@outputpage ...t {\vfil \color at hbox \normalcolor
                                                  \hb at xt@ \textwidth
{\@theh...

\@opcol ...lumn \@outputdblcol \else \@outputpage
                                                  \fi \global
\@mparbottom \...
<output> ...specialoutput \else \@makecol \@opcol
                                                  \@startcolumn
\@whilesw \i...

\newpage ...prevdepth \fi \fi \vfil \penalty -\@M

\enddocument ->\@checkend {document} \newpage
                                              \begingroup \if at filesw
\ifnum ...

\end  ...ook {env/#1/end}}\csname end#1\endcsname
                                                  \@checkend
{#1}\expandafte...
l.6 \end{document}

?

-----------------

When switching to the 2019 installation (using macOS TeX Live Utility)
lualatex and pdflatex compile the same file without any error message.
Neither dinbrief.cls nor xcolor.sty have changed between 2019 and 2020,
thus it must be something else, but I can't find the reason.

Older stackexchange-entries point to problems with \AtBeginDocument,
but neither dinbrief.cls nor xcolor.sty use that.
This is not limited to macOS, we observed the same error on Linux.

Any help is appreciated
  Detlev
-- 
Detlev Droege, Uni Koblenz, FB Informatik | Fon:+49 261 287-2769
AGAS, Computational Visualistics          | Fax:+49 261 287-100-2769
P.O.Box 201602, 56016 Koblenz, Germany    | Email: droege at uni-koblenz.de