I’m using the 1.0beta12 release and I have QoS enabled. I just tried a profile database backup and it worked fine.
By your description of the problem (starts, but doesn’t finish) I’m inclined to think this is a browser or network issue and not a particular ZS issue.
Have you tried backing up from different browsers or a different PC?
I’m using Firefox 3.x which seems to work fine. Maybe give that a shot if you’re using something else.
If you’re using IE 8, there are a lot of compatibility issues with numerous sites. You might want to play with IE 8’s compatibility mode settings (also make sure you have the FULL compatibility mode addon pack installed).
Anyway, that’s what came to mind when you described your problem. I’ve seen that a lot here. Keeping browser versions and settings compatible with all the sites and portals we use is a constant hassle.