Some common errors include the following:
XenServer failed to create snapshot on VM. Xen error: The specified storage repository has insufficient space
SR_BACKEND_FAILURE_44 insufficient space
Failed to provision new VM. The specified storage repository has insufficient space
This is the standard message XenServer gives when it cannot allocate enough space on the SR hosting the VM to create a Snapshot. Its not the Backup target it is complaining about, just the SR hosting the VM. A Snapshot requires an equal amount of space to that of the VM, though only temporarily, while the back up is running.
When you encounter this error, it’s a good idea to confirm the problem by manually taking a snapshot of the afflicted VM in XenCenter. If this fails with a similar error, then you will need to find a way to free space on that SR. Typically this can be done by deleting any unnecessary snapshots, removing unwanted virtual disks, etc.
If you want to learn more about the post-snapshot deletion process, potential bottlenecks in coalesce, and how to identify and address coalesce related issues in a XenServer environment. you may want to refer to the following Citrix KB article -> https://support.citrix.com/support-home/kbsearch/article?articleNumber=CTX201296.
Workaround
A workout around is to let Xackup Halt or Suspend the machine and also turn off Snapshotting in the Job configuration. This might be acceptable for you. The alternative is to increase the size of the SR or to let XenCenter run a scan to clean up unneeded space.