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Ok... I've already partially solved this problem. It turns out that there were 2 users named Anonymous in my database. I deleted one of them, and the error disappeared.
Yeah!!
Now a new problem though... When I click the link to enter the admin page, I get a 404 page not found error even though I can verify that the files are there. This is more confusing to me than the original problem. Anybody have this problem before?