I had something similar happen to me this morning.
This followed on from yesteday when I all of a sudden had an issue with LiveID & OpenID authentication. I manually disabled both of these in the DNN Authentication table. I then mistakenly set the "LogoffControlSrc" field in this table for the DNN setting (AuthenticationID = 1). I then found that I couldn't logout.
I resolved this by setting the LogoffControlSrc = '' where AuthenticationID = 1
Not sure if this will fix your problem, but it might be worth checking.