My Bitcointalk account keeps logout even if I click that remember me box when I close the Bitcointalk tab and turn off WiFi.
This happens because you logged into your account using another device or browser during that time when you closed your first bitcointalk tab. That's why they always ask you to login again when you open the browser and Visit the Bitcointalk link.
The bad news could be that someone else using another device is accessing your account without your knowledge. Try to check your account's IP address history
@OP, I think the error message that you get is when you are on the middle of creating a thread while your account get logout due to inactivity. This happened to me many times because I’m very slow to create a thread.
That error is usually different from the one stated by the OP.