Just recieved another update from veeam...
Regarding the general earlier errors:
Warning : Failed to use CBT: [Task e75c4247-2ee1-7b51-088c-b970ace60f3d (Async.VDI.list_changed_blocks) failed: . SR_BACKEND_FAILURE_460. . Failed to calculate changed blocks for given VDIs. [opterr=Source and target VDI are unrelated]
Our QA team did hear back from Vates support, who says its tied to a recent bug they've found with blktap calls: https://github.com/xapi-project/blktap/pull/444
Our QA engineer hasn't heard back yet on ETA when it will be pushed out in a general release yet though