It depends. If you sign in to this site and then try to go to another hosted forum, let's say Predator4-Movie then you may need to re-login. However if you go to Scified and then Predator4, it will sync up all your logins. I always sign in on Scified and then go to whichever forum because Scified syncs you up to all sites on the network.
If you've tried that and you still get logged out:
1) Check how fast your browser flushes cookies. If you clear cookies often it will log you out with each cookie clearing.
2) Upgrade your browser / use a more modern browser like Google Chrome. Microsoft Edge and Safari don't always cooperate as well but the site should log you in properly on all browsers.