I figured it out. You're not allowed to section a configuration that has been exploded within the assembly file. Why they didn't say "Cannot create a section view from an exploded assembly configuration" is beyond me. Why you can't section an exploded assembly configuration is also beyond me.
Here's a new one to me. I have a drawing with a number of views
> already existing. These views come from a variety of files. I have
> recently designed a new variety of the assembly shown, which is saved
> in a different file (i.e. it is not a configuration). I'd like to
> show a section view of this assembly so I first enter a "front" named
> view. When I start the section view tool and draw the section line I
> get the following error:
>
> Cannot create a section view from a "Detail" view
>
> Since this is a named view, something is amiss. I get the same error
> in a fresh drawing too, so it seems to be linked to the assembly file.
> Any ideas? Anyone else seen this?