Addendum:
... it works fine with TerraMaster where I have have defined C:/Users/<myname>/Documents/FlightGear/TerraSync as target folder. Haven't tried TerraSync yet. At least the console tells me:
terrasync-dir = C:/Users/<myname>/Documents/Flightgear/Custom Scenery/TerraSync
scenery-search-paths =
C:/Users/<myname>/Documents/Flightgear/Custom Scenery
C:/Users/<myname>/Documents/Flightgear/TerraSync
C:/Users/<myname>/Documents/Flightgear/Custom Scenery/TerraSync
I suppose there still is a mistake, but I don't have a clue how to change terrasync-dir.
My fault? Bug in FG?