The SURFdrive web-interface will send the user to his organisation to login, after a successful login the user returns to SURFdrive and the error appears.
Solution:
The user doesn't have the SURFdrive-attribute. This can be verified via profile.surfconext.nl. Go to to the 'Profile' tab and look at 'Entitlement'. There you should see, a.o. the following attribute:
x-surfnet:surf.nl:surfdrive:quota:100
For users who get the 'unauthorized'-error, this attribute will be missing. The servicedesk of your organisation is in controls of these attributes and can add this for you. Afterwards the user should have access (after a restart of the session). It is important that the user tries to login first via web-interface (not the client).