Read ECONNRESET error during "Retrieving the source data"
The sObject I'm fetching has around 800k records and when this error occurs the command stays in progress, but doesn't do anything.
ECONNRESET typically can say 3 things:
Internet connection suddenly interrupted.
Remote server stopped to respond (Salesforce environment maitenance is in progress).
Remote call timeout issue due large data and/or slow internet connection.
The request timeout for the query is set to 4 min. For very large data and slow internet connection you may face variety of timeout issues.
Try to reduce the amout of fetched records by using WHERE clause.