Well, I've tried everything including http://www.mediamonkey.com/forum/viewtopic.php?t=7362 's "must synch to root e.g <Artist>-<Title>" and variations, to no avail.
I wonder if that official mesage's listing of "Sandisk Sansa" as a known-to-work USB Mass Storage Devices applies only to the Sansa eXXX series, not my mXXX.
So I've given up and switched to MTP mode (losing open access to the storage), in which playlist sync does work.
V3.0.3.1164 Sansa m250 playlists are empty
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