Executing this command in ssh console works. Afterwards the library is completely available again.
What does that mean now? As mentioned in my first post, 'Wait for network before starting Kodi' is enabled. Is LibreELEC not waiting long enough?
I just saw that after wait for network there is now a 'Maximum Wait Time'. It was set to 10 seconds. I changed it now to 30 seconds and it seems, now it's working. I think this max wait time option was not there in older LibreELEC versions (or I just didn't saw/recognized it).
Anyways, thanks for the great support. Very happy to have it back working