I tried lots of settings, re-started FCXE810 several times with no success.
then I realized, that FCXE does NOT write all settings to Freecommander.ini -- I corrected the Freecommander.ini manually and everything was ok.
just some pics to make it clear:

then I changed some entries in FCXE-settings dialogue

QuickViewer did not work as expected, so I examined Freecommander.ini and found some strange behavior

by the way:
I also found weird font size on QuickViewer right-click menu (I've got 15" screen, 2880x1620, FontScaling 200%)
