I can confirm that using ZxMig and ZXBackup is THE way to do a Zimbra-to-Zimbra migration. It takes a while of waiting, but in the end everything will be transfered the right way with the right permissions on shares. Just make sure you do the checks mentioned in the "Preparing the migration"-part. If a problem is found, and not solved, it can happen that ZxBackup will halt/stall on importing one or more of the mailboxen causing you to restart the import process excluding the already done mailboxes and the problem mailbox.
ZxMig is also a nice and free way of preparing for disaster recovery ;)
thanks a lot for your help! :)
now I'm able to transfer all of our mailboxes on the designated date :)
It works perfect now.