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.
See the following thread for details.
Edited, Aug 16th 2013 1:37pm by svlyons
Lyonheart, like Eorzia, will be reborn in FFXIV!
FFXI veteran (Lyonheart and Lakiskline of Lakshmi)
1/467 on signed HQ Weskit!!!