"System.Net.WebException: The underlying connection was closed: The connection was closed unexpectedly."
This error message is received when trying to do a test connection to the SharePoint site collection.
See the screenshot of the error message.
The SharePoint Server does not have the Notes Migrator for SharePoint Web Services installed and configured.
1. Install the Notes Migrator for SharePoint Web Services on the Web Front End
2. Verify the Central Administration page has the "Configure Notes Migrator for SharePoint".