OK, HTML Preview showed me the exception that the red box contains:
An error has occured while processing GroupSection
The relation is not parented to the table to which
this DataView points.
What does this mean? My data source consists of a single table and all the fields referenced in the conditional formatting expression are members of that table. Here's my condition, which is the same as it was in the snapshot above:
=Sum(Fields.orderHandling + Fields.orderShipping + Fields.orderTaxes)
OK, I just tried this with the Sum() function removed, and the exception went away! That makes no sense. I need to test against the sum of the expression over all the rows in the group, not against a single row. So there needs to be an aggregate function.