Thank you for replying.
I did not find anything at /var/media/XXXX at first.
I run the USB-disc thru Windows chkdsk.exe - it 'repaired' some flags.
I re-inserted the cleaned disc on the RPI4, and immediately the disc showed up
in the menu of the 'FileManager', and indeed also in /var/media/XXXX.
Problem solved.
I did not encounter issues with the in-kernel driver NTFS3,
but out of curiosity I went looking in the add-on repos for NTFS-3G.
Couldn't find it, but it seems that "LE Add-ons 12.2.0" is already running om my Kodi,
so I assume the fuse-driver NTFS-3G might already be installed.
Anyway, no problems anymore - thanks.