This particular error indicates that there is already an entry on the server with the answer 'mirko' to the question 'Q1'.
Entries which are incomplete cannot be uploaded. When adding a new entry, you need to answer ALL its questions. If you exit the form halfway through and save (to complete the entry later) the entry is marked as "incomplete" and it will get a warning yellow icon (like a minus sign).
Entries which have some required questions not answered cannot be uploaded.
This usually happens when a question gets set as required at a later stage. The form gets updated on the app correctly but the user already has some entries on the device he cannot upload anymore, as they are missing the answer to the now required question.
The user needs to amend the old entries and fix the errors. Let's see an example:
The project above is a simple two questions form asking for name and sex. As you notice, the "Sex?" question is not required. Let's add it to the device and add an entry:
Before uploading the entry, we go back to our form, set the "Sex?" question as required and save the project.