Infopath 2007 and Project 2007 PSI Webservice

[micfreak]

hi,

i want to use Forms Services to get the Projectlist from the PSI Webservice
of Project Server 2007. I have bound the Method "ReadProjectList" to a
DropDownListBox. But when i execute the form, the following message appears:

"Diese Abfrage kann auf das Objekt "ReadProjectList" vom Typ "DataObject"
nicht ausgeführt werden.
Die angegebene Abfrage kann von InfoPath nicht ausgeführt werden."

or english:

"The following DataObject either cannot be created or cannot be initialized:
ReadProjectList
The query cannot be run for the following DataObject: ReadprojectList
InfoPath cannot run the specified query.

Is this a security problem ?

Thanks for help