It's due to a botched Windows patch. The solution back during Phase 3 was to uninstall that patch. However, I've read that MS rolled out a fix recently. So if you haven't gotten Windows patches in the last few days, you should check.
Edit:
See the following thread for details.
http://ffxiv.zam.com/forum.html?forum=152&mid=1376658376138815150
Edited, Aug 16th 2013 1:37pm by svlyons