When starting the VMware vSphere Web Client on Windows Server 2012 R2, the following message is displayed: To view this page ensure that Adobe Flash Player version 11.5.0 or greater is installed. After that it's not possible to enable Flash because of the following message: Flash Player is integrated with Internet Explorer in Windows 8. You do not need to install Flash Player.
The solution for this is not that hard, you must install the Desktop Experience feature on the server. Let's start PowerShell for that, and type the following command: Install-WindowsFeature Desktop-Experience
After that it's needed to restart the server, and install the latest Flash update with Windows Update. (KB3001237)
Flash Player will be available now. Just try to start the VMware vSphere Web Client again, and you will be fine.
When ActiveX Filtering is enabled you may discover issues. Just turn it off in Options > Safety > ActiveX Filtering.