Upon attempting to shadow user's RDP session running on RD Session Host on Windows Server 2008 or Windows Server 2008 R2 the following error appears:
The issue is caused by Microsoft tools incompatibility: shadow.exe is being used on Windows Server 2008 R2 and lower and mstsc /shadow on Windows 2012 R2 and higher .
Should you need to shadow a user's session running on Windows Server 2008 or 2008 R2, RAS console must also be running on Windows Server 2008 or 2008 R2. If the RAS console is installed on a later version of Windows Server, shadowing will not work.
As a way around, you can install RAS Console component on separate Windows Server 2008 or 2008 R2, and then use this console to remotely connect to your farm and manage RDS sessions running on Windows Server 2008 or 2008 R2. Please note that to finish a remote control session, the administrator must log off from the RAS console remote session. This is a limitation of shadow.exe utility from Microsoft that doesn't take any arguments.