I ran into an issue with some Citrix servers in the environment. Users were getting a protocol driver error. After poking around I noticed ICA was not listening on port 1494.
It appears that this issue was caused by Microsoft patch MS12-024.
Known issues with this security update After you install this security update on a Windows Server 2003 SP2-based terminal server, Remote Desktop Protocol (RDP) clients or Independent Computing Architecture (ICA) clients may be unable to connect to the terminal server. To resolve this problem, install hotfix 958476. MS Hotfix 958476 is available here.
The following instructions correct the issue until the hotfix can be applied:
Identify that ICA is not listening on port 1494.
netstat -an | findstr 1494
Open the Terminal Services MMC
Disable then Enable ICA
Run netstat again to verify ICA is listening on port 1494.
Retest the Citrix connection.