I built a new Citrix Web Interface server recently and configured it connect to a XenApp 6 and a Presentation Server 4.0 server farm. The applications hosted on the Presentation Server farm displayed correctly, but when you tried to launch an application hosted on the 4.0 server farm, a 31203 error would appear.

Looking in the Windows Event Log, the following is seen:

The farm OLDFARMNAME has been configured to use launch references, but a launch reference was not received from the Citrix XML Service. Check that the farm supports launch references or disable launch reference requests. [Unique Log ID: 7394119b]

I found a Citrix error message PDF of some sort and for the 31203 error it stated:

The farm has been configured to use launch references, but a launch reference was not received from the Citrix XML Service. Check that the farm supports launch references or disable launch reference requests.

The solution this PDF describes is as follows:

To use launch references, all the servers in the specified farm must run XenDesktop or Presentation Server 4.5 or later. Citrix recommends that all servers in a farm run the same product and version. If the farm is running XenApp 4.0, with Feature Pack 1, for UNIX or Presentation Server 4.0 and earlier, ensure that the RequireLaunchReference parameter is set to Off and that OverrideIcaClientname is set to On in the XenApp Web site configuration file, WebInterface.conf.

To resolve the issue, I changed the RequireLaunchReference parameter on the XenApp 6 servers webinterface.conf file.

1. On the Web Interface server, locate the WebInterface.conf file and open it with NotePad.

2. Locate the following entry around line# 169:

RequireLaunchReference=On
Replace it with the following entry:
RequireLaunchReference=Off

3. Save the WebInterface.conf file and test.