VSS error code VSS_E_OBJECT_NOT_FOUND

I’m using Urbackup client 2.2.6 on a Windows 2008 server and Urbackup 2.2.11 on a Ubuntu server machine.

From many months I’m not able to backup the Windows machine. I’ve always a VSS error code VSS_E_OBJECT_NOT_FOUND error.

This is the debug:

    2018-11-05 22:51:47: Removing VSS log data...
2018-11-05 22:51:47: rc=0 hasError=true state=0
2018-11-05 22:51:47: ClientService cmd: #IiTWrsHaqZ1ajI7Ji8nLS#WAIT FOR INDEX async_id=bff41c19ffdce8e82d71b080bb82bc19#token=ktKvNZGkJpJC4rRyCpN6
2018-11-05 22:51:47: Wait for async index bff41c19ffdce8e82d71b080bb82bc19
2018-11-05 22:51:47: Final path: C:\Backup
2018-11-05 22:51:47: Script list at "C:\Program Files\UrBackup\backup_scripts\list.bat" does not exist. Skipping.
2018-11-05 22:51:47: Deleting files... doing full index...
2018-11-05 22:51:48: Final path: C:\Backup
2018-11-05 22:51:48: Removing deleted directories from index...
2018-11-05 22:51:49: Number of Writers: 7
2018-11-05 22:51:49: Writer SqlServerWriter has failure state VSS_WS_STABLE with error S_OK.
2018-11-05 22:51:49: Writer ASR Writer has failure state VSS_WS_STABLE with error S_OK.
2018-11-05 22:51:49: Writer Shadow Copy Optimization Writer has failure state VSS_WS_STABLE with error S_OK.
2018-11-05 22:51:49: ClientService cmd: STATUS#pw=g6n09DPHQ4KSFG7cqiyV26KteCyBOL
2018-11-05 22:51:49: Writer Registry Writer has failure state VSS_WS_STABLE with error S_OK.
2018-11-05 22:51:49: Writer BITS Writer has failure state VSS_WS_STABLE with error S_OK.
2018-11-05 22:51:49: Writer WMI Writer has failure state VSS_WS_STABLE with error S_OK.
2018-11-05 22:51:49: Writer COM+ REGDB Writer has failure state VSS_WS_STABLE with error S_OK.
2018-11-05 22:51:49: Preparing change block tracking reset for volume c: failed: Funzione non corretta. (code: 1)
2018-11-05 22:51:49: Preparing change block tracking reset for volume unc\serversorriso\share failed: Funzione non corretta. (code: 1)
2018-11-05 22:51:50: ERROR: backupcom->AddToSnapshotSet(&(Server->ConvertToWchar(selected_vols[i])[0]), GUID_NULL, &additional_refs[i].volid) failed. VSS error code VSS_E_OBJECT_NOT_FOUND
2018-11-05 22:51:50: ERROR: Indexing Windows components failed
2018-11-05 22:51:50: Script "C:\Program Files\UrBackup\postfileindex.bat" does not exist
2018-11-05 22:51:50: Async index bff41c19ffdce8e82d71b080bb82bc19 finished with "error - index error"
2018-11-05 22:51:50: ClientService cmd: #IiTWrsHaqZ1ajI7Ji8nLS#GET VSSLOG
2018-11-05 22:51:50: VSS logdata - 235 bytes
2018-11-05 22:51:50: rc=0 hasError=true state=0
2018-11-05 22:51:50: rc=0 hasError=true state=0
2018-11-05 22:51:57: rc=0 hasError=true state=0
2018-11-05 22:51:58: ClientService cmd: #IiTWrsHaqZ1ajI7Ji8nLS#2LOGDATA 1541454718 0-1541454708-Starting unscheduled full file backup...
2-1541454711-backupcom->AddToSnapshotSet(&(Server->ConvertToWchar(selected_vols[i])[0]), GUID_NULL, &additional_refs[i].volid) failed. VSS error code VSS_E_OBJECT_NOT_FOUND
2-1541454711-Indexing Windows components failed
2-1541454711-Constructing of filelist of "dati_uno" failed: error - index error
2-1541454718-Backup had an early error. Deleting partial backup.

Any hint in order to solve the problem? THanks