Hi Ian,
Have you recently deleted a course category? If so, it could be a browser cache issue - do you have the same problem if you use a differenet browser or clear the browser cache? If not, as far as I can tell course/index.php is getting this value from the variable $CFG->defaultrequestcategory - which must be populated somehow, but I do not know.
But to be honest, this is really a usability issue and not to do with course formats, you may have better luck in the General Problems forum.
Regards,
Gareth