Hi Chethana,
did you recently update the mendix version of your application? I had this message when I upgraded and needed to refresh my cache.
Otherwise have a look at the application log or the console log of the browser when this happens.
As a workaround, you can try the followings:
When the page opens, check whether the user is already authenticated (for example with a nanoflow using getCurrentUser). If the user is logged in, immediately redirect them to the home page. This way, even if the user navigates back using the browser’s Back button, they will be automatically sent to the home page and won’t be able to interact with the login page again.