Only affects OG content type - have validated the issue by deactivating and reactivating OG Menu.

Popups Add and reference works fine when OG Menu not installed (http://drupal.org/project/popups_reference).

After instaling OG Menu, error message "Unable to open: http://dev.musicosity.com/node/add/artist?"
This content type is an OG, but other content types ok, so it is assumed a conflict with OG types.