To fix this issue, please do the following:

1) Start CloudBerry Backup Service under administrator account: go to Tools > Change Service Account, enter local administrator account name and password, click OK. The service will be restarted automatically.

It is generally recommended to use local administrator's account name because domain administrator is not always a local admin on the host machine.

2) Make sure that VSS is enabled for partitions you are backing up and check the option "Use system VSS provider".

Having the same problem but could not locate the "Use system VSS provider" option for image based backups. Located it on the Advanced Options page with an additional click on Show Advanced Disk Options.

Thank you Ted, mine was stuck at 99%. Your settings worked for me.