Sorry for my late reply I was on vacation.
I added the folder via "Zeroconf browser" and via "Windows network (SMB)". In both cases I have full access to the folder and the files in it. As a test, I loaded Test.avi, Test.mp4, Test.mp3 and Test.mkv into this folder.
Test.avi, Test.mp4 and Test.mp3 are played. Test.mkv and "Grill-den-Henssler- VOX2020-08-3020-15.ts" are not played error see first post from me. Both Kodi systems use the same "SMB: //" setting.
As a test, I set up an ftp share for this folder, with this share all files on Libreelec are played. Only not with SMB.