I had similar problems in my workplace - the Network object would not be created (permission denied when I debugged) but Infopath reports no error when the form was opened - just an empty field.
I have 2 solutions I used - get user to enter their login name into a field - then use the 'IsCurrentUser' function of Infopath to validate that what they put in really is their user name - don't forget to put the domain name for your workplace at the beginning for the 'IsCurrentUser' function.
The other way was I had an SQL server with a 'view/stored procedure' which had only the 'SUSER' variable as a value. By referencing this view as a secondary datasource and retrieving the values when the form opens - I immediately make the current username available to the form in the secondary datasource field. If you have an SQL database you can use in this way - I think this is the nicest method.