Scan Library files returns empty



I am new at using Ubuntu (16.04) but have PlexMediaServer running on an old Windows Server 2008 box. That installation was quick and painless. Ubuntu give me more headache :-(
Finally everything running but when adding library folders I can not use the Browse function, the folders with movies does not show up. I enter the path manually and click 'scan', nothing found.
I checked and set the file and folder properties to User:Plex The Plex Group contains the User and the Plex user installed by the software. Info of a file: (-rwxrwxr-x 1 username plex 1977304850 feb 25 11:00 Xxxxxxx.mp4).
I think this is not the problem, but I ran out of options.

Any directions where to look/find?

Thanks in advance!



(PlexmediaServer, Ubuntu 16.04.4 LTS)