Well this has really nothing to do with the cookie prefix. That's just how the cookies are named. They are still assigned to the domain.
I'm not sure if the cookie script can be modified. I'm not the real cookie-fixing expert. I just am very familiar with the problems. I took a look at the code and it confused me.
But you'd have the same problem here if you tried to login at
https://www.ubbdev.com/threads/php/login.phpThen clicked to
https://www.ubbdev.com/threads/php/ubbthreads.phpYou'd be logged out.
Here's something you might try. If you were to give a new unique cookie prefix.... then everyone would be logged out and everyone would have to log back in and get new cookies.
That might fix your problems. I think.
Just tell people to update their bookmarks and don't try to login at old URL's etc...
That might help a great deal.