No execute permission on XmlPort with ID 50002

Hi all

We are encountering a strange issue when trying to access a NAV web service. The following is displayed in the browser:

<s:Envelope><s:Body><s:Fault><faultcode>a:Microsoft.Dynamics.Nav.Types.Exceptions.NavNCLNoExecutePermissionException</faultcode><faultstring xml:lang="en-GB">No execute permission on '' XmlPort with ID 50002.</faultstring><detail><string>No execute permission on '' XmlPort with ID 50002.</string></detail></s:Fault></s:Body></s:Envelope>

In order to access this web service we are using a test user windows account who has been set up within NAV 2009 R2 with SUPER rights. Using the domain administrator account does not generate such an error and the xml code is displayed correctly. But we do not wish to use the domain administrator account for obvious reasons.
What we cannot understand is that using this test user account we are able to access other NAV web services without any issues at all! So I cannot understand why this is happening.
What additional rights do we need to grant this user and has anyone encountered this issue before? Incidentally, adding this user to the local Admin group of the NAV server does not solve the issue either.

Any comments/help on this would be much appreciated.

TIA
Pierre