Thank you, Ludens.
I'll watch out for your suggestion, spending long time in a topic.
However, the problem still persists when I don't log out.
Because I usually don't. For days/weeks.
I usually leave my browser open, and never log out. The board recognizes me as Blodrast, because I just hit refresh/reload (to see the new posts, etc), and I'm not asked to relogin, and I still have "Welcome, Blodrast" on my screen.
Perhaps it periodically deletes the cookies anyway ? (they do have a limited lifetime...) but then, it doesn't delete the login-related cookie, apparently.

hmm.
I'll watch more carefully to see when it happens. It happened to me today, but I had not spent a long time in a topic (1-2 minutes tops)...