Starting an incremental backup results in an error

I run an Windows Client.
The Windows Client have an Volume D:
This Volume is an integrated ISCSI Volume in Proxmox

The Server is an urbackup Server based on Debian.
But when backing up this client, there is always an error on drive D:

The error Mail was:
UrBackup just did an incremental image backup of "fileserver".
( 3 infos, 0 warnings, 2 errors)
2018-11-15 00:20:14(info): Starting scheduled incremental image backup of volume "D:"…
2018-11-15 00:20:14(info): Basing image backup on last full image backup
2018-11-15 03:37:12(error): Error on client occurred: Error while reading from shadow copy device (2). Die Anforderung konnte wegen eines E/A-Gerätefehlers nicht ausgeführt werden.
2018-11-15 03:37:13(info): Time taken for backing up client fileserver: 3h 16m 58s
2018-11-15 03:37:13(error): Backup failed

On the Windows Client the Error in the Event Log was:
The data could not be moved to the transaction log. The data may be corrupted: Volume ID: “D:”, Device Name: “\ Device \ HarddiskVolume4”.
(The I / O device reported an I / O error.)

Die Daten konnten nicht in das Transaktionsprotokoll verschoben werden. Die Daten sind möglicherweise beschädigt: Volume-ID: “D:”, Gerätename: “\Device\HarddiskVolume4”.
(Das E/A-Gerät hat einen E/A-Fehler gemeldet.)

Is there a solution?

Your disk is damaged. You have lost data (if it wasn’t backed up previously).

No. This is not the fix for this error.
I already did that. There no errors
The Windows Client is a Virtual Mechine.
And the Backup was very slow

But why is the backup so slow?