How can I tell why backups not working?

I’ve installed Better WP Security in five different WordPress instances successfully. The backup function on just one is misbehaving exactly as nemof describes. Its twin is working just fine. Both these installs are multisite enabled and only accessible on our intranet. We’re running WP 3.51 on RHEL 6.3 with shipping Apache configured with multiple virtual hosts, and IUS php 5.4.13-1. The site whose plugin isn’t able to do backups is behind a BigIP LTM. We’ve checked to make sure that everything else is identical: file system permissions, php.ini settings, etc.

Can someone suggest some diagnostic procedures we can follow to find the root cause of this problem? Any assistance would be much appreciated.

Thanks Handoko, at least we know we’re not alone! Of course at my shop we already had a fairly easy to use backup solution in place: a perl script that does a mysqldump every night and that can be kicked off at will by my developers from a webmin console. At this point my concern isn’t so much the loss of backup functionality in an otherwise stellar plugin, but whether it’s a symptom of a problem with my platform or WordPress configuration. Around here we like to uncover root cause as a way of heading off future trouble (which is always just around the corner!).