Get webadmin-access with users from root-domain

Dear Forum,

we've purchased a brand new ASG120 and successfully joined it into a sub-domain. Now we have the problem that we can't log in (webadmin) with an account from our root-domain. In the sub-domain, we've created a domain local security group and added the users from our root-domain.

I have tried three different ways to get access:

- username
- root-domain\username
- username@root-domain.local

none of them seems to work. When we add an account from the sub-domain into this domain local security group, the authentication works fine. Does anyone have an idea what the problem could be?

Failure from the aua-log:

2012:04:24-10:49:34 ASG aua[1105]: id="3005" severity="warn" sys="System" sub="auth" name="Authentication failed" srcip="10.***.***.***" user="root-domain\USERNAME" caller="webadmin" reason="DENIED"

Thank you very much

David