Hyper v host image error

i cant´t backup the hyper v host:
error in server:

Info 15.07.19 04:43 Starting scheduled full image backup of volume “C:”…
Fehler 15.07.19 04:43 Request of image backup failed. Reason: Creating shadow copy failed. See client log file for details.
Info 15.07.19 04:43 Transferred 207 bytes - Average speed: 1.928 KBit/s
Info 15.07.19 04:43 Time taken for backing up client HYPER01: 3s
Fehler 15.07.19 04:43 Backup failed

client log:

2019-07-15 17:09:00: ClientService cmd: STATUS DETAIL#pw=hQTOIxIgHneeqdRjIUvT5YfnwpkO36
2019-07-15 17:09:01: ClientService cmd: STATUS DETAIL#pw=hQTOIxIgHneeqdRjIUvT5YfnwpkO36
2019-07-15 17:09:02: ClientService cmd: STATUS DETAIL#pw=hQTOIxIgHneeqdRjIUvT5YfnwpkO36
2019-07-15 17:09:03: ClientService cmd: STATUS DETAIL#pw=hQTOIxIgHneeqdRjIUvT5YfnwpkO36
2019-07-15 17:09:04: ClientService cmd: STATUS DETAIL#pw=hQTOIxIgHneeqdRjIUvT5YfnwpkO36
2019-07-15 17:09:05: ClientService cmd: STATUS DETAIL#pw=hQTOIxIgHneeqdRjIUvT5YfnwpkO36
2019-07-15 17:09:06: ClientService cmd: STATUS DETAIL#pw=hQTOIxIgHneeqdRjIUvT5YfnwpkO36
2019-07-15 17:09:07: ClientService cmd: STATUS DETAIL#pw=hQTOIxIgHneeqdRjIUvT5YfnwpkO36
2019-07-15 17:09:08: ClientService cmd: STATUS DETAIL#pw=hQTOIxIgHneeqdRjIUvT5YfnwpkO36
2019-07-15 17:10:23: ERROR: Not found
2019-07-15 17:10:24: ERROR: Error creating Hyper-V snapshot of . Output: C:\Program Files\UrBackup\hyperv_snapshot.ps1 : Das Argument kann nicht an den Parameter "VmName" gebunden werden, da 
es sich um eine leere Zeichenfolge handelt.
In Zeile:1 Zeichen:31
+ .\hyperv_snapshot.ps1 -VmName ''
+                               ~~
    + CategoryInfo          : InvalidData: (:) [hyperv_snapshot.ps1], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationErrorEmptyStringNotAllowed,hyperv_snapshot.ps1
 

2019-07-15 17:10:24: ERROR: Creating shadowcopy of "hyperv:///C:" failed.
2019-07-15 17:10:24: ERROR: Creating shadow copy failed. See client log file for details.