Known reasons for this to occur:

1. 1. You bookmarked the Federation Sign-In URL instead of the application's URL, You may have logged in using an invalid link or bookmark. Please check the link and try again.
2. 2. You attempted to reach multiple sites requiring authentication at the same time, close your browser and only open one site at a time to prevent this issue.
3. 3. Your authentication process took too long, ensure you have sufficient network bandwidth and complete your authentication in a timely manner.