After opening a case with NetApp support service I have finally solved this issue.
The solution is to modify (or add if we don't have it) a registry key named RemoteBackupThreadTimeout located in HKEY_LOCAL_MACHINE\SOFTWARE\Network Appliance\SnapManager for SQL Server\Server, that is used as timeout for remote backup threads. This key is a DWORD and indicates number of minutes. Its default value is 480 (minutes).
Click Start, and then click Run on the SMSQL server.
Type regedit, and then click OK.
Click the '+' sign next to 'HKEY_LOCAL_MACHINE' in the left pane of the Registry Editor window.
Navigate to the below registry entry: HKEY_LOCAL_MACHINE\SOFTWARE\Network Appliance\SnapManager for SQL Server\Server
Add DWORD (32-bit) value named 'RemoteBackupThreadTimeout' and set it to a number larger than 480 (default value in minutes).
There is no need to restart SMSQL service after this change.
By the way as the verification job failed I have the database attached, the vmdks linked to the vm, the datastores presented, and the volumes cloned. Do I have to revert all this automatic work made by SMSQL manually?