Hi Rolf,

The example file ``eso-ex1.tex'' of eso-pic package
does NOT work fine with latex-dev (LaTeX2e 2020-10-01).
See attached.

Ulrike kindly told me the reason:
https://chat.stackexchange.com/transcript/message/55660212#55660212

> \@tempdimc doesn't have the value the code expect as the
> length is now used internally too and so changes.
> The example shouldn't use such temp lengths, if it can't
> be sure that other code interferes.

Actually, `\@tempdimc` is newly used in the definition of
\put etc., since LaTeX2e 2020-10-01.

I think you can replace \@tempdimc e.g. by \my@tempdimc:

\newdimen\my@tempdima
\newdimen\my@tempdimb
\newdimen\my@tempdimc

and use them in the calculation in ``eso-ex1.tex''

Best regards,
Hironobu Yamashita

\documentclass{report}

\usepackage{eepic}
\usepackage{showframe}

\begin{document}
This is a test.
\end{document}

\documentclass[oneside]{memoir} %oneside!
\setlrmarginsandblock{0.5in}{2in}{*}
\checkandfixthelayout
\usepackage{showframe,lipsum,color} %showframe uses eso-pic
\AddToShipoutPictureFG{\AtTextUpperLeft{\textcolor{red}{XXXX}}}
\begin{document}

\lipsum[1-10] 

\newpage 
Suggested fix:
\makeatletter
\renewcommand\AtTextUpperLeft[1]{%
    \begingroup
      \if@twoside  %<-- added
      \ifodd\c@page
        \ESO@tempdima=\spinemargin
      \else
        \ESO@tempdima=\paperwidth
        \advance\ESO@tempdima-\textwidth
        \advance\ESO@tempdima-\spinemargin
      \fi                              
      \else                    %<---
       \ESO@tempdima=\spinemargin %<----
      \fi %<----
      \ESO@tempdimb=-\uppermargin
      \AtPageUpperLeft{%
        \put(\ESO@tempdima,\ESO@tempdimb){#1}%
      }%
    \endgroup
  }

\lipsum[1-10] 

\end{document}