Please see below for an explanation of Warning signs that may be found next to volumes listed under the Volume section of an Agent's Summary Tab.
By design, if volumes have <30% ( less than 30% ) free space, a warning sign will be displayed beside those volumes and the summary of volumes. Hovering your pointer over the warning sign will display "Some volumes have not enough free space. Please, pay attention that transfer may be failed for full volumes."
This does not affect the backups from failing unless it doesn't have enough free space for it to accommodate a snapshot. VSS snapshots generally require 10 -15 % free space on a volume to take a shadow copy.
This warning can be safely disregarded as long as the backups are completing successfully.