by Howard Gaukrodger.
SOLVED - in my case...
I had almost the identical problem, but with a different file - locallib.php - so I thought I'd share my experience.
Environment: Win Vista, FF 18, Moodle 2.3.1, PHP 5.3.
On saving a new course, the screen would display the white screen of death. Debugging identified the locallib.php file to be missing from the /enrol directory. On copying this file into the /enrol directory, everything was fine. Easy when you know!
What I don't understand is why the file wasn't there in the first place.