"A login process is already in progress for your current session" occurs if you clicked on a link that required a login but closed the tab without entering your username and password or the tab is still open. Since you clicked on a link that required a login, it started a session and that session needs to be completed or terminated to allow the login prompt to occur.
To resolve this issue you can follow the on screen instructions or completely close your web browser and try again. If the issue persists, clear your browser's history and cache.