by sonia sonia.
Hi, I have the same Issue and the solution (with moodle 2.9.1. installed) was check all the options in general backup settings except: comments, bagdes,logs,histories and unchecked. The locked option in all of them unchecked too...and then all works fine, all it's backup in the correct way. My sharing cart with troubles too.
I hope it's useful for all of you.