Here are the results:
- iTunes Match is off on all devices, except on my mac@home housing the initial library
- Then I delete all the duplicate playlists with the aforementionned applescript
- All seems OK on the mac (but I haven't tried to turn Match off and then on to check if the playlists are really gone).
Then, back at work, I open iTunes on my iMac, and add it to iTunes Match.
After 2 minutes, the 50000 playlists appear ... again ...
Just for testing the sync ability of Match, I have also created a new (non-empty !) playlist on my mac@home. This new playlist does appear on my mac@work. So if I create a new playlist, sync works fine, but if I deleted the 50000 empty playlists from iTunes, they still appear on another mac ...
I have now deleted the duplicate playlists from my mac@work too, with the same applescript. Everything seems fine, but I am pretty sure that the problem will be still there I turn Match off and then on again, whatever the device (mac or idevice).
Next step: trying to add my iPad to Match.