I don't think there's enough people on this list so a request for help "with Windows and in German" will work. I'm German, but not a Windows user so for now I've translated the email, if someone knows a solution I can translate that back. I'm also asking questions below (in German) suggesting to "pip install lxml", asking about installation sources for Inkscape and Python and whether this error was always there.
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First thing is the error message. The error seems to stem from (looking at my own Inkscape) the line
inkex.py:113: from lxml import etree
They're using Inkscape "09.1" (assuming that's supposed to be 0.91?) on Windows 10.
The pictures show they're using the Interpolate extension, with settings Exponent 0.0, Interpolation steps 7, Interpolation method 2, Duplicate endpaths True, Interpolate syle True, but if I understand the email correctly this happens with all extensions. (Makes sense since it doesn't seem to be anything specific, pointing back to inkex.py.)