Failed to calculate changed blocks for given VDIs

If your Xen backup job fails with the message "failed to calculate changed blocks for given VDIs", the most common culprit is that Xen is unable to process your changed block data and return it to you due to an internal error or problem on the Xen side.

If you run the job in TRACE (tools->settings->logging->"Enable trace level...") you will see additional messages confirming this:

Then the job will fail as follows:

While it is possible this issue is specific to a single SR and could be worked around by migrating to a different one, the root cause of this issue is unfortunately a hypervisor issue and cannot be addressed by Quadric Software.