If CRM 4 Client is installed on a Citrix/Terminal Services server and the initial installation (done when in CHANGE USER /INSTALL mode) also includes starting Outlook for the first time then a registry key is set in the Terminal Services registry shadow. This means that once you go into CHANGE USER /EXECUTE mode and a new user logs in they get the registry keys set during installation as part of their profile.
This is by design if the registry keys that are set are not needed to be unique per person. And CRM has one registry key that needs to be unique for each user logged into the terminal server. This is the ClientRemotingChannel registry value located at HKCU\Software\Microsoft\MSCRMClient.
If more than one person is connected to the terminal server and running Outlook with CRM configured then behind the scenes the CRM hoster application will be running. The ClientRemotingChannel registry key controls the ability of this application to communicate with Outlook and the CRM server and therefore this registry value must be unique for every logged in user on the terminal server at the same time. If more than one user has the same value (which they will if initial installation is done as above) then the 2nd concurrent user will fail to connect to CRM via Outlook – web access will work fine.
Therefore ensure that the shadow registry value for this (HKLM\SOFTWARE\Windows NT\…\MSCRMClient) is not set and that all users that already have a duplicate value have the registry value deleted. The hoster application will recreate the registry value when it starts with a unique value if the registry setting is missing.
A clue to the existence of this error will be the application event log error "Failed to create an IPC Port: Access is denied".