This is the demo users from which I am logging into
But why it is creating problem only in dataview not in datagrid?
With the details provided above I can only think of one possible reason for your issue. Before that I have some assumptions that may have caused this,
1. So my assumption is that you have a Custom MF linked to Create Users and in that Microflow you are creating an Account object for your StudySiteUsers. When creating an Account Object a Username and Password needs to be provided and cannot be empty (Reason being Account is Generalized from System.User)
2. The reason why you are able to Add/Edit StudySiteUsers in DataGrid is cause may be you are using the default Save button generated in New/Edit Page (Assuming) so here the Account object is not being created but only the StudySiteUsers object.
Hope this makes sense.