I really did spend some time researching this and was surprised to not find any answers (this great thread doesn't address this problem at all.... I'm trying to recover from my unfortunate situation: I set up a secondary user on my Nexus 7 4.2.2 and my device is not (yet) rooted. Now I like many others realise that rooting will wipe everything in order to unlock (if I had only read that first), and I'm trying to backup all app data for the secondary user.
When I try to run any adb-based backup on the secondary user, it just hangs and then times out, creating an empty .ab file. The owner's user account backs up just fine.
Any ideas? Is the multi user just too new? I have run into other annoying things with it (no VPN client will connect, can't get into developper settings, can't even change the TIME settings, etc...)