InfoPath Dev

Dear Hilary, thank you for the response. I checked and everything is checked, so this unfortunately can't be the reason.  

I am having a problem that is driving me insane, since I am using the same approach in quite a few other InfoPath forms (in the same site collection) without problems: I need to set a Person Picker to a specific person. For demonstration purposes I put the following three steps on a button where I set the DisplayName, ...