I got this error after update to 2.0.3 from 1.12.
In “Visits overview (with graph)” I see “Error: The report ‘nb_downloads’ was requested but it is not available at this stage. You may also disable the related plugin (nb) to avoid this error.”
This error occurs after I view a site w/o any statistics.
When I go to sites w/ statistics and readd widget again. So I can see graphs. But this is one time solution.
PS! I can provide an admin access to my piwik site for developers. Just PM to me.