As to the cause of the error, I concur with Artie it was most probably a temporary glitch in your system that was cured by the universal computer panacea — a reboot.
I don't know if this works all the time, but logging out/in can solve problems as well. I have done this a few times rather than restart, and it takes less time.
The problem with logging out/in - which has historically been the go-to troubleshooting step - is that if it doesn't work, you've got to restart, and if that doesn't work, you've got to shut down/start up, because each option gets deeper into the system than the previous one.
I find it easiest to skip the preliminaries and go right to shut down, and its now taking only a minimal amount of time, as opposed to the 1-2 minutes it used to take, is a plus.
