make sure that the Source and Destination VLAN id's are the same, if this is not possible, power off the Guests and cold migrate them,
if this is not working, manually copy them from one host to another with something like Veeams FastSCP. with this you would need to unregister from your host and then move the files, then register on the new host.
Without access to the logs it will be difficult to get to the root casue of your issue.