We are having a very similar problem. The difference is that in our case the backup process stops at a different point: when it says "zipping files". As in your case, no error message or explanation of what has caused the problem. I've posted a message in the general problems forum but so far nobody has given me any solutions:
http://moodle.org/mod/forum/discuss.php?d=49633#227315
One person suggested that it could be the name of the course (if it has spaces or special characters). That doesn't seem to be the problem with our courses. One of the courses that is failing to backup doesn't have any space in their name and there are plenty that do backup well that contain spaces in their names. Another person suggested that the problem could be due to having run into "resource limits" but I don't know what these resource limits could be. Let's see if someone can tell us what's wrong.
Josep M.
Backup and restore
Course Backup Hangs Up
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