Just signed up with WPMU, first plugin installed after DashBoard and get this error. I signed up after hours of looking for the right solution and need to get this running tonight.
Had to deleted folder: /wp-content/plugins/marketpress/
to free up admin/plugins.
Reinstalled, same issue. The error (at least last part of URL) is as follows:
Fatal error: /marketpress/includes/admin/class-mp-orders-admin.php on line 822