PowerBI/Powerquery - "Received an unexpected EOF or 0 bytes from transport stream" while 'Applying Changes'

Question

I have a powerBI that grabs data from 4 outlook mailboxes from Microsoft Exchange Server. Each mailbox is a separate query and they each have identical columns.

With these separate mailbox queries, I append the data from each mailbox into 1 new query and then expand the Sender field into Sender.Name and Sender.Address and perform a pivot.

I believe the total number of records is under 50,000, but Powerbi takes multiple hours to apply changes only to eventually return an error Received an unexpected EOF or 0 bytes from transport stream" . What is happening here?

If I delete all the steps in the Appended query, the changes apply successfully after several minutes, but I do require the additional steps unfortunately.

EDIT: if I just Append instead of Append as New, it seems to work without errors. I want to end the day on a high note so I dare not try refreshing it again.

Answer 1

Appending to existing query rather than appending to a new query yielded no error message. Explanation: unknown.