by Ken Task.
If a backup is successful, there will be a 0 byte size .log file. If the log file is larger than 0 bytes, it does contain a log of the plans it was executing 100 -> 1000 (1000 means success - which means you'll never see that one cept in one case ... when the built .mbz backup of the course couldn't be copied to the destination directory.
IF that temp directory contains an .mbz file, the backup was successful, but the setup/system has a hickup in using 'copy' to get the .mbz file to designated directory.
Progress bar in this last case might get to 99.x% and then appear to freeze.
But you say the one course is getting to a % less than that, so ???? that's why debug and checking of error logs.
Thought of one other one might see in raw logs .... 'server as gone away'. That is really a MySQL setting for max_allowed_packet ..
https://dev.mysql.com/doc/refman/5.5/en/packet-too-large.html
Then there is also an overall description of your setup ... web server we know has the moodle code (your's has a proxy), but what of DB server ... same server as moodle code or is it a dedicated/shared DB server? How about location of moodledata? That on an NFS .... that also could be the reason for some 'large' backups failing.
So ... rather than guess any more ... debug info please.
'spiirt of sharing', Ken