Unfortunately I don't know what might be causing the issue you're seeing. What I'd recommend is to get a sysdiagnose from one of the devices that's showing this issue and file a bug report so that we can take a look at what's going on. You can get the debug profile and instructions for how to gather a sysdiagnose from here:
I'd suggest grabbing the sysdiagnose in the middle of the hang. If you can post the bug number I'll see if I can get it looked at to get some insight into what might be going on.
much appreciated. The bug # is 30670398