If you are a personal subscriber, the number of users set to your account is one and if this has been exceeded you will see this error message. If this does happen, you can reset and log out of any other browser by going to the 'reset logins' option that will appear.
If you are a subscriber at an institution, the number of users is set to an agreed limit and when this has been exceeded you will see this error message.
Other common reasons for being locked out:
Users closing the browser and not using the 'log off' button.
Inactivity for more than 20 minutes logs you out. For example if you are reading text and not using your mouse, the system may see this as inactivity.