Both times this has happened to me there has been very little free diskspace on rootfs. This is not through me having used it up, as with normal use I have 40MB free. So this space is taken up by something temporarily. Last time, I didn't even reboot and the space was freed. My non-scientific working theory is that this space actually dropped to zero, and hildon-desktop wanted to write something to update the widgets, this failed, and the entire widgets data was lost.