Hi Ulrike,
> It compiles again if one remove the \MakeUppercase from \sectionmark:
>
> \makeatletter
> \def\sectionmark#1{%
> \markright {{%
> \ifnum \c@secnumdepth >\m@ne
> \thesection\quad
> \fi
> #1}}}
> \makeatother
Thanks for the report. We already found this issue earlier this week.
It seems to be caused by the fact that TeX4ht redefined \MakeUppercase
and \MakeLowercase. This redefinition was related to accented
characters, and it was quite old code. A recent upgrade to LaTeX
kernel broke this code, so I removed it. Your code seems to work fine
with current TeX4ht sources, but not with the version in TeX Live.
Best regards,
Michal