Thank you for your quick answer, I see, that I mixed up ControlContents and FormFields.
Thus, I have to ask my question in a different way: I want to add in a protected document a ControlContent with a macro, so that i can put some text in the ControlContent afterwards. The code of the macro would be:
If ActiveDocument.ProtectionType <> wdNoProtection Then ActiveDocument.Unprotect
Set objCC = ActiveDocument.ContentControls.Add(wdContentControlRichText)
objCC.Title = "Comments"
objCC.SetPlaceholderText , , "Comments"
Application.ActiveDocument.Protect wdAllowOnlyFormFields, Password:=""
Obviously this does not work. Thus, the question is now, can I protect the word file with vba in such a way, that the user can put text into, e.g. RichText-ControlContent some text?