A couple of possibilities: account corruption is an obvious one, but before you go out and reinstall everything try the following -
1) Repair permissions
2) Go into disk utility and check if the app can see the disk or not. If yes, try and repair it or whatever.
That's what Apple siad to me yesterday when I reported a similar problem. Did not try it yet. Keep me posted!
Bop 'til you drop
MB Air 13 | i7 | 8 Gigs of shiny DDR RAM | 512 blistering SSD
iMac 27 i5 16Gig 1TB
iPad Air 64Gig
Time Capsule 1TB
iPhone 5: "It's fast..." & a bunch of ill fitting ear buds
Canon 7D proper camera with nice L glass and Lightroom 4