Open Windows Services and restart the Yosemite Server Backup service. Also stop and start the YSB software on the client end and then the master server.
Please be aware that the Yosemite Server Backup software is a product that has reached its end-of-life in 2017. The release notes for 10.6 I find here: https://campus.barra.../release-notes/ indicate support up to Windows Server 2016, Windows 10. It also no longer supports versions of Red Hat 4 or earlier. This would include similar release numbers for CentOS. I can vouch for the 10.4 version still working across Windows Server 2088 R2, 2012 and 2012 R2. As I am on an older version I have never tried Windows Server 2016 with it. If you are trying to use it with Windows Server 2019, Red Hat 8 or CentOS 8, you are running YSB software on an operating system that it has no idea of.