When using the xDB Data Migration tool, Sitecore analytics data might fail to migrate from Sitecore XP 8.x to Sitecore XP 9.x with an unhandled exception. The issue occurs if xConnect does not respond within a certain time (more than 30 seconds by default). You can find the following exception in the log records:
An error occurred while sending the request. (pipeline: Process Single Contact from MongoDB Pipeline, pipeline step: Add Contact Model to xConnect Batch, pipeline step identifier: 46ebebbf-250f-4bc2-a5f2-e4a64b58e561)
A task was canceled. (pipeline: Process Single Contact from MongoDB Pipeline, pipeline step: Add Contact Model to xConnect Batch, pipeline step identifier: 46ebebbf-250f-4bc2-a5f2-e4a64b58e561)