New-Moverequest / user in subdomain

Hi there,

I'm migrating from a mixed Exchange 2007/2010 environment to Exchange 2013. The Domain contains a root, where the Exchange servers are placed. Users are spread through different subdomains. When I now want to migrate a user from a subdomain using the Exchange 2013 EAC, I get an error.

The operation couldn't be performed because object ''ef22e4fb-d633-47fd-9cf0-f7f44ba8125d' couldn't be found on 'DC.rootdomain.local'.
    + CategoryInfo          : NotSpecified: (:) [Get-MailboxDatabaseCopyStatus], ManagementObjectNotFoundException

How can I move user mailoxes, which are located in subdomains?

Thanks.