Migration Assistant errors occur while transferring information "Migration unexpectedly quit. Please try again later.”
Source of backup:
- iMac 24" mid-2007
- macOS El Capitan 10.11.6
- Time Capsule 3rd Generation
- Time Machine backup
- connected to network router with Ethernet
- MacBook Pro 13" without Touch Bar mid-2017
- macOS Mojave 10.14.2
- connected to network with Wi-Fi
After starting the new MacBook Pro with a network restore to install Mojave, initiated Migration Assistant for the initial configuration, sourced from a Time Machine backup. Everything was selected for transfer which was the default selection. After waiting for the calculations to complete and initiating the transfer and several hours of progress, the assistant showed this error: "Migration unexpectedly quit. Please try again later.”
A different scenario leading to the same error message is described in another question.
Why does it happen and what should be done about it?