Just been there, when I chose to log out to see how something displayed from outside the forum, i.e. the latest activity button.
When I logged back in again, everything worked as normal, so I was certainly logged in OK.
Latest Activity button enforced log-off each time I tried it.
F5 didn’t cure anything, but clearing cookies and cache, just related to this site, from my browser cured the issue.
Opera on Windows 10 desktop.
Bill