This KB will assist you to enable logging in order to troubleshoot issues like backup failing.
1. Log as superuser/administrator
2. Go to CONFIG MGMT > SETTINGS > Adapter Logging
3. Check Log both DEBUG and FATAL messages + Enable logging of adapter operations to a file
4. Now after the issue occur you can review the logs in the DGE/DGEx that is monitoring the device, under <Traverse_Home>\apps\netconf\osgi-config\scratch\logs