Full image backup failed (Internet backup)

running urbackup-server on Debian Bullseye. Configured for internet backup I’m trying to backup a Windows Hyper-V virtual machine. The backup is running some time and fails with error “Request of image backup failed. Reason: Opening filesystem on device failed. Stopping.”
Can someone give me advice where to look further or how to debug the problem ? I think the backup is running and internet connection stops on one site. After that the backup is never tried again, but I can not confirm this theory.
Urbackup “client” on server version is 2.2.20-hyperv on Server 2016 Standard.
Is it possible to split initial backup to transfer the parts so transfer will not begin from start at large images ?