I suspect the -50 thing might actually be a red herring. I ddi try to take some bits and pieces of advice but nothing.
So right now, iTunes has the checkbox to sync music cleared - ie, don't sync music.
Despite this, the bottom status bar shows s blue bar and if I mouse over it shows 274 songs.
iTunes on the iPad still thinks that the songs are there, but if I try to play a playlist it just skips through every song trying to play it till it gets to the end.
-50 error is gone as I say.
Did update to 6.1. No difference.