Windows Server Backup reboot server

Hi everybody,

I've a 2008 R2 guest inside a 2008 R2 Hyperv host.

Guest execute a scheduled backup to a network share.
Almost at backup end (I can see 60 GB written on the network share out of about 70 GB total) the virtual machine shutdown and reboot.

No error is recorded in event log (just "unexpected shutdown" (ID 6008) and "restart without clean shutdown" (ID 41))

Sometimes, instead of rebooting, the server sits there unreachable from the network, and I must force shutdown from the host.

When the server reboot everything is working fine, until the next backup...

Regards,
  Corrado