Run following command via Exchange Management Shell to grant Full Access permission (change the admin user accordingly):
Get-Mailbox -ResultSize unlimited -Filter {RecipientTypeDetails -eq 'UserMailbox'} | Add-Mailboxpermission -User admin@company.com -AccessRights fullaccess -InheritanceType all -Automapping $False
If the problem persists, check following KB article and make sure that all required permissions are in place.
https://support.quest.com/metalogix-archive-manager-for-exchange/kb/4274943/what-permissions-are-needed-for-the-metalogix-superuser-account