is the "m" correct ?
Yes, it is - wanted to show the full actual path just in case.
Since the library is distributed among more than one disk, I use the disk letter as a suffix on the path.
Breaking news
Tried to enter MSIF path from the web interface and succeeded.
Next, checked on the LibreELEC device and it was there: has been correctly stored.
Thus, it seems that the problem may be circumscribed to x86/64 10.0.2 user interface.