I can login successfully to the website and have submitted posts in the past to the excellent PowerShell Q&A Forum from this same Windows 7 SP1 laptop [Internet Explorer 11], yet when I click on that forum today, the Forum posts are displayed but the Sign In icon appears in the upper right hand corner and the bottom of the webpage says I must be logged in to submit a post. Hitting the Internet Explorer back arrow returns me to the website’s home page, and my login name replaces the Sign In icon. If I then click on the PowerShell DSC Forum, it displays the posts and keeps me logged in, displaying my login name in the upper right hand corner and the Submit field at the bottom of the webpage. Logging out of the website, rebooting my computer, then logging back into the website, I still get logged out whenever I pick the PowerShell Q&A Forum, but only that forum.
This has happened to me as well. It is a browser caching issue. Hit refresh and you will see yourself logged in and all will be well.