I didn't reply for a long time because the synchronisation randomly worked the next day. But now I'm trying to sync again and I'm having the same issue.
So... iTunes tells me there's not enough space to synchronise. I have 25GB of music in my library, 20GB of which is on the device from my last sync. I want the 5GB of new music to be transferred onto the device. There's 18GB free space.
As you suggested, I tried to unsync all music. When I unchecked 'sync music', the blue audio portion of the storage bar dropped to zero, but when I clicked 'Apply', it went through a syncing process in which the original blue audio section just reappeared.
Now the 'sync music' box is unchecked, with 25GB of music still showing up on the usage bar and when I click 'sync', it tells me there's not enough free space (despite the grey portion of the usage bar reading 18GB). It says I need another 7GB, which would be about right if it was trying to add my full library to what's already on the device rather than just syncing the new stuff.