Hm... I used to get the same error, though that was from me logging in, not from trying to get into the User CP. The reason it did it for me was because I had a different skin set as my default (The Bottom of the Sea one, actually). I think you need to change the skin back to the forum's current default (The Dark Illusionist one). Maybe you could try changing it back using the thing at the bottom of the page, then see if you can get into your User CP?
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