Both guids explain the same thing. The only problem I have is that no mail user is created when a linked user is synchronized to the cloud.
The two users are made Linked Mailboxes by the command "Set-User -LinkedMaster...". When the user is synchronized without a link, it shows up in Exchange Online under Contacts and I can start a migration batch with this user.
The user also exists only once in the cloud, as it should, since it is linked by ObjectSID and msExchMasterAccountSid.
So I can't create a migration batch in which the user should be migrated. Unfortunately the migration batch can't find the user...
Did I make a mistake when setting up the Azure AD Connect or what could be the reason?