Ok, I figured out why "mvn clean" was failing. And yes, it was a stupid
mistake on my part.
(For those who care about the details... I have maven set up to
create the repository inside the workspace, but to make that work
an environment variable needs to be set. It wasn't being set when
I ran "mvn clean" by hand, although my nightly build script was
setting it. Oops.)
Still, that doesn't explain why the build failed to begin with.
Of course, building from scratch works.
Sorry to contribute to the noise...