Thanks everybody for your comments.
I'll do some tests with smbclient and see how that behaves.
I remember that this DID work in the past with the default software (WeTek Play box). But that crashed a year ago and the filesystem was broken. So I re-installed the lastest supported LibreElec (9.0.2) and here these softlinks (=NTFS Junctions) do not work any more.
BTW: on Windows I create such a Junction to another drive or directory with this command:
mklink /d /j c:\anydir t:
This creates a junction "C:\anydir" which is showing drive T: in that subdir.
--
I did a test with smbclient and everything is working there!
LibreELEC:~ # smbclient -U xx //xxx/yyy
Enter WORKGROUP\xx's password:
Try "help" to get a list of possible commands.
smb: \> dir
. D 0 Sun Feb 16 20:17:17 2020
.. D 0 Sun Feb 16 20:17:17 2020
!A D 0 Sat Oct 26 15:22:34 2019
!B D 0 Sat Aug 17 19:50:41 2019
So both junctions are visible (one drive is connected; the other is not);
In Kodi only !B is visible (the drive NOT connected); !A is missing.
so I checked whether smbclient can see files/directories of the next level (which are on the linked drive A):
smb: \> dir !A/
$RECYCLE.BIN DHS 0 Sun Jan 14 11:56:24 2018
movies D 0 Sat Jan 25 20:34:35 2020
System Volume Information DHS 0 Fri Nov 17 18:05:33 2017
music D 0 Fri Sep 6 17:14:26 2019
misc D 0 Mon Oct 28 21:24:13 2019
244189887 blocks of size 4096. 90550586 blocks available
Yes; smbclient is workiing with that share in the way it should do. It sees all directories and files down the tree.
But Kodi is not.
any idea how to investigate further?