I am having the EXACT same problem. Windows 2012 R2 Hyper-V + SCCM 2012 R2 (all hotfixes installed). It's driving me nuts. I'm about to pay Microsoft for a tech support call because I am dead in the water. I have Googled to infinity and beyond, and it seems no one else seems to have this problem. I'm trying to develop an image for a remote client using VPN to access their server. I don't have access to physical machines to try to capture from, hence why I'm using Hyper-V machines. When using an image capture media (ISO mounted in Hyper-V), it seems to start scanning the volumes, but when it moves to capturing the volumes, the machine just reboots without warning or any messages. I'm left with a very small WIM file on the network share (ensuring network permissions are correct) and the machine boots into OOBE mode. Anyweb - I have triple checked the network account and file share permissions. I have tried different network accounts on different file shares. I have tried Fixed and Dynamically expanding virtual hard disks, fixed and dynamic RAM. The SMSTS.log file on the drive shows nothing useful. I can see entries when it was performing sysprep, but the last entry in the log does not show anything related to the image capture. The last entry in the log has a timestamp that is 4-5 minutes before the image capture fails and reboots, so I don't think the actual image capture process logs anything. Unless it's in another log file that I'm not aware of.
This problem is making me very sad