Hi,
When you get a mail for every user subscribing to your list you've probably setup a notification for that. You can check your notifications in Configuration->Subscription.
Can you post the summary of your Automation?
As for the migration issue I suggest you raise a support ticket here: https://www.acymailing.com/support/. Please include login credentials for your backend so we can have a look art it. It would be helpful to add some detailed information in your ticket about some of these users you have issues with, so we know what to look at.
You're using an outdated Acymailing version. Please update to the latest version.