trouble accessing a site’s dashboard in multi-site mode

Hi,

Having some unusual trouble with the multi-site feature I haven’t been able to sort out…

The problem is that I can’t seem to get to the dashboard to secondary sites as anyone other than the superadmin.

Here are the clues, using mainsite.com as the primary domain where WP3.0 is installed and addonsite.com as an added site added.

1. We are using the WordPress MU Domain Mapping plugin by Donncha O Caoimh. This seems to work as all the sites are visible after we mapped the domains correctly.
2. You should be able to access our add-on site’s dashboards from the SuperAdmin >> Sites screen, but for all three add-on site’s the backend link goes to mainsite.com/wp-admin/
3. If you try to go to addonsite.com/wp-admin/ directly it redirects to addonsite.mainsite.com/wp-login.php and logging in from this page simply redirects you to addonsite.com/?dm_gotoadmin=1&back=/wp-admin/ which just shows the site’s homepage.

Oddly, we have another installation of WP3.0 with the same configuration that works as expected.

Ideas?