This may have nothing to do with AD authentication, but I have never seen it before - only in the one newly created portal with AD authentication. I created a new DNN security role, and then went to grant that role to users who had logged on to the site via windows credentials.

I get this error each time ... though it appears that the users were successfully put into the role.

ActiveTabName: Security Roles
RawURL: /forms/Admin/SecurityRoles/tabid/41/ctl/User Roles/RoleId/4/Default.aspx
AbsoluteURL: /Forms/Default.aspx
AbsoluteURLReferrer: http://............./Admin/SecurityRoles/tabid/41/ctl/User%20Roles/RoleId/4/Default.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.04506.648)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: dc76c1d5-33c3-4e29-a241-5d83cd869be9
InnerException: String reference not set to an instance of a String. Parameter name: name
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Globalization.CultureInfo..ctor
StackTrace:
Message: DotNetNuke.Services.Exceptions.ModuleLoadException: String reference not set to an instance of a String. Parameter name: name ---> System.ArgumentNullException: String reference not set to an instance of a String. Parameter name: name at System.Globalization.CultureInfo..ctor(String name, Boolean useUserOverride) at DotNetNuke.Security.Roles.RoleController.SendNotification(UserInfo objUser, RoleInfo objRole, PortalSettings PortalSettings, String Action) at DotNetNuke.Security.Roles.RoleController.AddUserRole(UserInfo objUser, RoleInfo objRole, PortalSettings PortalSettings, DateTime effDate, DateTime expDate, Int32 userId, Boolean notifyUser) at DotNetNuke.Modules.Admin.Security.SecurityRoles.cmdAdd_Click(Object sender, EventArgs e) --- End of inner exception stack trace ---

Any thoughts or should I ask elsewhere?