Request of image backup failed. Reason: Opening filesystem on device failed. Stopping

Hello

I use the Hyper V Client Trial Version for testing. I am very happy with it but suddenly it stops backing up with this failure

Fehler 16.02.24 07:08 Request of image backup failed. Reason: Opening filesystem on device failed. Stopping.
Fehler 16.02.24 07:08 Backup failed

Does anyone have an IDEA?!