@witziege
Our Privacy Policy is I think quite clear on this topic. It's actually being re-written as we speak to make it even more clear (should be out within a week).
All your files stored on our nextcloud are encrypted with a key created based on your credentials. This means that unless we do MITM attak on you and get your credentials we cannot see the content of the files. However, contacts, calendars, circles, tasks are unencrypted so potentially we can read it.