Polix, there are a few settings under "Security and Privacy" settings that might cause this. One setting explicity says this setting makes it impossible to remain logged into more than one computer ("Reset member's log in key upon each log in?") which was also changed and enabled. However, another setting stores the IP in the session (cookie) which also must be making it impossible to keeping logged in at multiple computers ("Match user's IP Address during session validation?") which wasn't changed but is still enabled.