Mercury is a session based application. When you login via the Mercury Login page, a session is started using the external ip address currently assigned to your internet provider's service.
If a second external ip address starts a session using your Mercury credentials (CAxxxxx) then it will start a new session and the original session is expired. You would be presented with the Mercury Session Expired message and returned to the login page.
The above could happen via the following ways.
1. Two people logged into Mercury using your account from two different locations.
2. You are connected to two separate internet services. Example, your iphone hotspot and also your home or office wifi.
3. You are connected to an internet service which has more than one external ip address. Common examples are public wifi services in Airport Lounges or Shopping Centres.
The Mercury Helpdesk team can check the logs to confirm whether multiple ip addresses is the issue. Just email the team on firstname.lastname@example.org for assistance.