Thanks reecardo.
This works fine, as long as I run in the debugger. However, once the workflow is published to the IIS, the component always returns the values of the Workflow Server and not of the user who is actually running it. I also tried to enforce authentication via the web.config and IIS settings, but it won't change that behaviour.
Do you have a hint, how I can force the component to get values from the computer of the user, who is actually running the workflow and not always from the server?