Wish I could tell you why ... specifically. Truth is, there are too many factors (potential causes for failure) to list and even then, none may be correct in every case. So you found no clues in those places, how about server logs themselves? PHP timing out? Not enough memory allocated? How about space? ... a bug in the backup of an add-on mod/block in those courses? Have you 'watched' the backup process via 'watch' the moodledata/backup/temp/ directory? See what I mean. :\
And Jolly, Jolly to you and yours too!
'spirit of sharing', Ken