The features File Portal and Web Manager of VMware Horizon Mirage both need Microsoft .NET Framework to work properly. While the File Potal still uses .NET Framework 3.5 SP1 the new Web Manager needs .NET Framework 4.0.
Sometimes if you want to access one of these consoles right after the installation you either get a HTTP 403 error (Web Manager) or HTTP 500 error (File Portal).
Both of these errors most often occure when the relevant .NET Framework version isn’t registered properly with IIS (Internet Information Service).
To solve this issue just run one of the following commands:
Mirage File Portal
Mirage Web Manager
These commands register the corresponding .NET version with IIS.
This workaround was only tested on Windows Server 2008 R2 with the 64-bit versions of the Mirage File Portal and Web Manager.
At this point I want to thank my colleague Christian Gehring for pointing out this problem and his help solving it.