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I left my client open (on the profile view page).
I left it alone for several hours.
When I came back, I hit the 'view profile' button again.
A message came up saying that I had timed out - that I needed to login again.
The client closed. Instead of restarting, an error box appeared stating that there was an unexpected error.
I started up the client again, to make sure that I didn't need to download any updates -- client was still up to date. servers were online. I was able to log in fine.