I had the same issue last week and erased all content, and re-sync'd, and things worked again. It's starting up again.
If I right click on most of my Zune Pass music, I see:
DRM: Yes - license expires 8/4/2009 (no sync rights, no burn rights)
WTF good is it if my license is active but I can't sync it? These stupid DRM tricks have got to be fixed. All I ever see are kludgy work-arounds and no updates.