The server address you first logged into is stored in your browser's memory. Any attempt to login again will be directed to the server address you first logged into. In order for you to login to a different server, the browser's memory needs to be erased. You MUST totally close down your browser to clear the memory, then login again. This will usually solve your login problem.
Sessions expire after several minutes of idle time.
This is done in order to reduce server load and to protect your privacy in case you forget to logout at the end of a session. When composing long emails, we recommend that you use the Save Draft button often, in order to save your work. This will prevent you from losing any modifications made to your messages and will help in preventing your session from expiring. In order to continue an expired session you need to login again. I hit a button and nothing happens.