Mostly this issue would be caused by the end user having multi monitors: http://support.citrix.com/article/CTX125693
This may be due to the change to the server, either by a removal of the farm, or a misconfiguration which causes a registry key to be deleted.
Follow the below steps to resolve the issue:
On the non-working server open registry key: HKLM\Software\Wow6432Node\Citrix\Install.
Look for a Binary entry called Setting. Note: If this entry is not present, shadowing will not work.
On a working server, open registry key: HKLM\Software\Wow6432Node\Citrix\Install.
Look for a Binary entry called Setting, you will see a binary number
Note the binary number on the working server and on the non-working server; create the binary entry called Setting at:
Enter the number from the working server in the non-working server
Note: Also you may get "Shadow failed. Error code 7044". You cannot shadow a session with multiple monitors enabled in Windows Server 2008 R2. Remote Desktop Services do not support shadowing multiple monitors.
To shadow a system running multiple monitors, they will need to be disabled in the Remote Desktop Connection application. To do this, uncheck the box for Use all my monitors for the remote session.