Solving 'the following layers were not correctly generated' in QGIS SAGA

I was working on a map recently and tried to use SAGA in QGIS to make a multilevel b-spline raster from some scattered vector points. (More to come tomorrow!) However, I kept getting the vague error “The following layers were not correctly generated.”

a screenshot of the error

I found some Stack Overflow posts with the same error but my problem seemed unrelated to the filename/disk space issues mentioned. As I experimented with the data, I discovered two circumstances that produce this rather vague error:

So if you’re encountering this, check that you’re interpolating a numeric field with a short alphanumeric name.