I've installed Portal onto an Amazon-hosted Server (not using the Esri AMIs, but just a regular Windows instance). The installation process seemed to finish without any problems and I was able to set up the Web Adapter along with a self-signed certificate for testing. The Portal is using the built-in Identity Store.
The problem is that I cannot log into the web interface. If I try to go to, say, https://localhost/portal/home/content.html, I get redirected to the Sign in page, enter my credentials, the authentication appears to succeed as the browser starts to load the content page, but then I am redirected back to the sign in page. I have tried logging in with port the portal administrator account that was set up during installation and a regular user account.
The strange thing is that I am able to log into the portal directory without a problem and can view the user accounts at https://localhost/portal/sharing/community/users/<username>. I can update the user account information and create new users from the directory as well.
The log files do not show any immediately useful information as to why the web interface is not able to keep the user's login active. Any suggestions?