I want to restore a web site backup on another ISPconfig server - same release - provided I've changed the corresponding DNS records and created the "empty" target site including databases, database users, protected folders etc. What else do I have to backup and restore manually to keep the restored web site certificates updated and the target web site virtually identical when used in browsers? Am I correct in assuming the backup and restore of an email domain and its mailboxes on another ISPconfig server is simpler and more straight forward?
There is for example this year old thread: https://forum.howtoforge.com/threads/move-site-to-other-server.93648/ I would say it is about the same complexity and amount of work. Your mileage may wary. If you find copying the mailbox files complicated, there is imapsync by Lamiral, https://imapsync.lamiral.info/ .
I just would not like to key in all mailbox passwords on the new server again and same sites files do not have same ownership usually, which complicates the restore.
So you want to migrate a site or email domain with mailboxes, when I understand it correctly. You can do this with the migration tool: https://www.ispconfig.org/add-ons/ispconfig-migration-tool/