I’ve been unable to use a CIFS storage location to work as backup storage for Urbackup in Linux (RHEL in my case). But that’s mainly due to a CIFS mount not having the ability to make symlinks and other FS trickery that UrBackup uses.
In my case i’m using a external backup NAS, such as a QNAP that I setup as a ISCSI target with a LUN for storage. This iSCSI Lun is then connected to my Linux UrBackup server, and formatted as ext4. At which point it becomes perfectly usable for UrBackup. Perhaps you can use a similair method?