Perhaps cookies can't be used on computers in internet cafe's (or just that one)?
If you can't log in, you can still post by entering your username and password when you make a post.
If you need to get into the admin area of your own forums but cannot log in, you can enter your forums' URL and add admin_home.asp to the URL. You'll be asked to log in into the admin section. Since that uses sessions instead of cookies, you should be able to use it.
You can also try to login without the Save Password box checked. Maybe that will fix the problem and it's probably safer anyway, since you don't run the risk of forgetting to logout when you're done.
Of course if you do not close your browser anyone using it afterwards can impersonate ya. You need to make sure that your logged in session terminates.