Answer
This has been associated with the Quick Report's result set simply being too large for the underlying XML-generation mechanism used for scheduled/emailed report output, and has been acknowledged as an already-reported platform limitation rather than something fixable through report or schedule configuration changes — if you hit this exact error, raising a support case referencing it (rather than continuing to adjust the report's own query or schedule) is the appropriate next step, since a fix timeline wasn't confirmed as of when this was last reported.