This is in Piwik version 1.10.1.
If you deactivate the UserCountry plugin, then try to track a campaign using the "pk_campaign" URL argument, this error shows up in the logs:
PHP Fatal error: Class 'Piwik_UserCountry_LocationProvider' not found in /srv/www/piwik/core/Tracker/Visit.php on line 665
The referrer does not show up in the "Campaigns" screen on the web interface.
Thanks for the report.
It seems to work for me now, maybe it has been fixed in git. please reopen if still not working with latest 2.1.1-beta