Backup always failed with VSS and SQLVDI errors similar to
SQLVDI: Loc=SignalAbort. Desc=Client initiates abort. ErrorCode=(0). Process=1276. Thread=3776. Client. Instance=X3INT. VD=Global\{532BEA15-7846-4C0C-86F7-49CA6A2E6E87}1_SQLVDIMemoryName_0.
Verified UAC, Windows Firewall, DTC... long story short, I've solved installing the Cumulative Update 2 for SQL Server 2012 SP2
Link to find most recent versions of SQL Server cumulative updates: http://sqlserverbuilds.blogspot.it
Cheers,
Marco