The following exception is being thrown when the number of attachments in the _Attachments folder defined in a migration job to SharePoint O365 exceeds 5,000 rows:
"System.Exception: Error adding document 'November 2012 Report.docx' ----> System.Exception: Error saving file to SharePoint ---> System.Net.WebException: The underlying connection was closed: The Connection was closed unexpectedly."
In the migration job, _Attachments, is defined as the destination folder for document attachments. Unlike the SharePoint Folder where you can map a dynamic name from Notes to the SharePoint folder to get past the 5,000 row SharePoint limit, the _Attachment folder name appears to not be modified at the job run time.
WORKAROUND:
You can migrate the attachments to an alternate library (remember that the library would have to allow more than 5000 items to meet the needed scenario requirements).
