Athena error: INVALID_FUNCTION_ARGUMENT Invalid format: ""

I am troubleshooting a long query with many joins, and getting the error in the title. I've found a part of the query that appears to trigger the error; it's a query that is joined to the rest of tables and subqueries with a CROSS JOIN, as the result is a single row of average values to be attached to every row in the full result set. The first of the averages makes use of to_unixtime(from_iso8601_date(date)) to calculate an average date. This average uses a subquery that explicitly excludes null and empty string values for the calculation of the average. This part of the query runs fine on its own. It's when that result is combined with other averages calculated from columns in the same source table that the error occurs. Originally, this first average and the others were all part of a single SELECT statement inside the CROSS JOIN; I've also tried separating that first average into its own CROSS JOIN, and keeping the rest of the averages (which also run fine on their own) in another CROSS JOIN. Same error though. This full query formerly ran without error; it's only in the last week or two that this has happened.