SingleOutputMapAlgebra_sa "succeeds", but no output

The same code works great on all of our computers, except one. It runs SingleOutputMapAlgebra_sa from a VB-scripted Add-In, and throws an error. I can find it in the ArcToolbox Results, and I can run it again from that prompt, where it says "Succeeded, elapsed time 0.0 seconds", and the output file does not exist.

I just went to the trouble to uninstall and reinstall ArcMap and Spatial Analyst, and it behaves exactly the same.

When I run Raster Calculator from ArcToolbox with the exact same input, it works.

Any clue?