https://inkscape.org/forums/beyond/not- ... own/#c5602I fix the problem!
And it's not what everyone repeats again and again! 
The problem is that windows has a bug (or is it a feature?) with the registry. If you install a font correctly - and this is to copy it manually to c:/windows/font - the necessary registry entries eventually are not created (correctly). And this will be a problem for Inkscape - because other programs show the fonts.
    So  remove all fonts from the windows-dir to a temp-dir (the system fonts can not be moved)
    donwload FontReg 
http://code.kliu.org/misc/fontreg/    unzip FontReg and move the Fontreg.exe to the temp font-dir
    start command line/shell: WIN+R and "cmd"
    change to temp font-dir
    fontreg /copy
    all fonts will be installed new and the problem is fixed.