BIG thanks @Zapf (works perfect for me: Inkscape 92.1)
and for harry people: in A to Z nutshell:
as a remember the plugin for Inkscape is available here: http://wiki.eleksmaker.com/doku.php?id=inkscape
after download an unzip => copy the files in the relevant folder (Inkscape) \share\extensions
edit the "Eleksmaker.py" line 3108 as per (Zapf printscreen above)
orientation_scale = 1 #3.5433070660
@njhollett Just extract the files from EleksMaker InkScape Plugin.zip and copy the extracted files in plugin folder to the inkscape\share\extensions\ folder. Maker sure you are not running Inkscape or you will not see it in the extension menu. Once done start Inkscape and you should see it appear in the extension menu. I hope this helps you.
I solved the problem for now.
You can see the result in the "Christmas theme laser engraving works awards collect 12.7 - 12.24".
I converted the jpg with Corel Photopaint to b/w-bitmap.
The conversion of the details was much better than with the path conversion from Inkscape.
I cutted the picture with the mask function in three pieces with the same hight and a small overlap.
There were no problems to open the bitmaps with EleksCam.
I had to choose the outline mode and set the same hight for all three pieces an the job was done
without further problems. the fit of the pieces was perfect.
Nonetheless it would be interesting to know what exactly happens in Inkscape when you cut out a
piece of a picture, save it as a new picture and there is still the whole original picture like a "invisible ghost"
in the background.
Perhaps we can get eleksmaker folk to post updated version of plugin with correct scaling value for newer inkscapes? I want to imagine the issue is on inkscape's end and some internal changes as to how scale works recently (at least from all my googling). I could test if the scaling works when reducing inches by the same ratio in a few days.