There was a long thread in the forums where the reported issue is:
E:\Piwik\piwik\misc\log-analytics>python import_logs.py --url=http://arf E:\logf iles2\Intranet\ARF_net_iis05\W3SVC769991110\ex140118.log Fatal error: Unauthorized
at least three users are discussing this issue. The solution is in this post
Enable Anonymous authentication in your IIS ... edit it and choose 'Application Pool' instead of a specific user. If this works then you can change the Anonymous authentication to the user of your choice.
PS. You can also enable anonymous authentication only to your piwik virtual directory.
Maybe we could detect this error and display the solution to user? or create a FAQ?
Issue was moved to the new repository for Piwik Log Analytics: https://github.com/piwik/piwik-log-analytics/issues