On both media servers also check that telnet to port 1556 on the client works, otherwise it seems to be communication problem possibly related to firewall configuration.
The key thing here is to check whether your file is actually included in the differential backup - use bplist -C <client> -k <policyname> -X -s <ctime> -e <ctime> -R / (get ctime from the image itself, <client>_<ctime>) or BAR GUI to do so.
I would ...
Has it ever worked?
Unplanned system restart has to be investigated on the OS level as suggested by PatS729, might be the system is going to a "Saving State" during the backup? If that is the case, make sure that Guest VM can be " quiesced" for onli...
According to https://technet.microsoft.com/en-GB/library/dn531029.aspx LIS are included for specified Ubuntu versions.
In addition to Marianne's comment, also check Event Logs on the Hyper-V node which "owns" affected VM in case BPBKAR32 process is ...