Magic ETL appears to run all the way to indexing, but fails with an un-specified “Server Error"

My ETL appears to run all the way to indexing, but keeps failing with an un-specified “Failed while executing....The server experienced an error". When I view the details of the "Run", each of the steps has "no additional details"

It is an extraordinarily simple: two inputs, group one, join both, output. I have used both datasets extensively so the data is very clean. The only possible wrinke is that one is 300 million rows.

Thoughts? What does "The server experienced an error" on indexing about?