Perhaps it's the "sudoku.com" in the address instead of "www.sudoku.com" that gives it the strange effect...
I'll post the link below, and also a version adding the "www.", and see if it makes any difference?
http://forum.enjoysudoku.com/viewtopic.php?t=5312
http://forum.enjoysudoku.com/viewtopic.php?t=5312
PS: Just checked it, the link without the "www." really gave me the impression that I was logged out, but when I went back to the original page containing the links and refreshed, I stayed logged in... So my guess is that the system in "http://sudoku.com" thought I wasn't logged in, while the system in "http://www.sudoku.com" thought I was...