Not so much a solution ...
I gave up waiting for 6.1 so updated and under v6 I can not replicate the problem. One of the good things about being old is not getting caught up in why's and therefor's. Best is to just be happy and move forward.
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I've found two other threads with different causes but same effect. I find it hard to believe that this situation has been encountered so seldom.
My files are located on a networked volume. Stuff happens and there have been times where the data folder needs to be rescanned. In such cases, once rescanned and everything is otherwise happily humming along, the playlists are empty. In the sidebar they are listed and the number of files purportedly in each is shown. But when a playlist is selected the main panel of the GUI says Playlist is empty. Talk about a major PITA.
In the first thread below there is this line:
playlists are located in the database file on the server and can be extracted
I can't find it and not sure I'd know what to do anyways.
viewtopic.php?f=2&t=16533&p=70867&hilit=empty+playlists#p70867
viewtopic.php?f=2&t=16247&p=69679&hilit=empty+playlists#p69679
Subsonic 5.2.1
Mac OS X 10.11.6
Any help/insights will be greatly appreciated!