The error message (remarkably void of instruction) confirms what you fear is correct... that the course has not been updated with your modifications. Meaning, however much time you spent on that course description... was lost (though if you click back now it still might be there). Minus the impression this grim error screen puts off, I was able to find an obvious solution.
I received the "coursenotupdated" error while trying to edit a course under the settings link. My particular problem was solved by shorting the course title, as well as the short name. It appears that in the php/html files a max length is specified for these fields (most likely for other fields as well ). It is also probable that in the MySQL database, Moodle has limits for storing string lengths for course titles, short names et cetera.
The easiest solution is to shorten your title, short name, idnumber or any other field where it seems likely you are pushing the upward limits of the form fields character quota.
Hope this helps!