Do you guys tick the remember me box when you login ? Try that and if it does not work clear cookies on your computer and try again.
It may also help if you actualy log out every so often.
Last week it kept logging me out every twenty minutes or so.
It was doing this on both of my computers. I cleared the cache on both computers, logged back in and it works fine now.
Interestingly it never effected access via Tapatalk on my smartphone.
Clearing cashe worked for me.