[racket] handin-server install problem: gnu-gunzip bad header

This semester I set up a handin server and all my students are using it
...except for one.

When he tries to install the prepared plt file via "Install .plt file",
DrRacket gives the complaint "gnu-gunzip bad header". Does anyone have a
pointer to what the problem might be?

He's using DrRacket v6.0.0 on a Mac.

The plt file installs successfully for all other students on a range of
Mac, Windows and Linux environments.

Thanks,

Breanndán