After authenticating successfully I would be presented with an Http/1.1 Internal Server Error 43531
After some Troubleshooting I could narrow it down to a specific Setting in my Session Policies for the NetScaler Gateway. If you used my previous Blogpost on how to configure the NetScaler Gateway for Storefront Remote Access you might also run into the same Problem.
After upgrading to the 10.5 NetScaler Build you need to edit your "Receiver for Web" Session Policy and move the Storefront URL from the "Home Page" Field under Client Experience to the "Web Interface Address" Field under Published Applications. You also need to enable "ICA Proxy" and set it to ON. Below are two Screenshots with the corrected Session Policy Settings: