Problem

While migrating to SharePoint/One Drive you noticed the following error in your logs:
“Error : [Target Connector] – The remote server returned an error: (429)”.

Cause

The error occurs, when you start too many users at the same time, and target system forcibly closes the connections due to throttling policy.

Resolution

Restart your migrations later. It is also recommended to start your users in the several batches (15 per 15 or 20 per 20), so this way you will not be throttled by Microsoft.