Thanks for the video, WarpCP. We definitely haven't ignored this issue and we have looked into it multiple times as it's due to a shared cookie issues.
I've sent this off to our web team again to do another pass at it as it was thought to have been fixed.
A temporary fix though is to click "Logout" when you have clicked the Sign-In button and it will remove your cookie and allow you to Sign-In properly with no issues.