As confirmation, it's all OK after today re-rebuild.
xo-web 5.67.0 bumped from xo-web 5.64.0

I've replaced /root/xen-orchestra/packages/xo-server/src/xo-mixins/backups-ng/index.js from master branch which was last edited 9 days ago with the file from rajaa-selector-locking-mode branch which semms the most recent.
At first attempts Backup job skipped with Job canceled to protect the VDI chain.

Following Backup troubleshooting on the host /var/log/SMlog was

Aug  8 09:19:07 a300 SM: [3856] lock: tried lock /var/lock/sm/bbea2d83-61ba-bf37-38bf-45e004cb5faa/gc_active, acquired: False (exists: True)
Aug  8 09:19:07 a300 SMGC: [3856] Another GC instance already active, exiting
Aug  8 09:19:07 a300 SMGC: [3856] In cleanup

Waited few minutes until

Aug  8 09:24:14 a300 SMGC: [3604] No work, exiting

then started the Backup job: it completed successfully!

Hi,

Thanks for all your effort on xcp-ng!

I run XenOrchestra (built from source) on a dedicated Debian 10.4 VM.
Maybe you have any hint on the VM backup error that I get after yesterday update of VM Debian and XO for the Delta VM Backup:

 server3 (a300)  
 Snapshot 
Start: Aug 8, 2020, 08:42:05 AM
End: Aug 8, 2020, 08:42:06 AM
 VM Backups 
 transfer 
Start: Aug 8, 2020, 08:42:06 AM
End: Aug 8, 2020, 08:42:06 AM
Duration: a few seconds
Error: handler.outputStream is not a function
Start: Aug 8, 2020, 08:42:06 AM
End: Aug 8, 2020, 08:42:06 AM
Duration: a few seconds
Error: handler.outputStream is not a function
Start: Aug 8, 2020, 08:42:05 AM
End: Aug 8, 2020, 08:42:07 AM
Duration: a few seconds
Type: full

Destination Remote is a NFS share.
Another backup job (Pool metadata and XO config) to the same Remote is working correctly.
I've created a dummy SMB share and configured as Remote, it has the same behavior.

xo-server 5.64.1
xo-web 5.64.0
XCP-ng 8.1.0 release/quebec/master/17

Error deails:

        {
          "data": {
            "id": "9695a80c-0d40-4819-a757-cc3f212f4fe8",
            "isFull": true,
            "type": "remote"
          },
          "id": "1596865326786",
          "message": "export",
          "start": 1596865326786,
          "status": "failure",
          "tasks": [
            {
              "id": "1596865326789",
              "message": "transfer",
              "start": 1596865326789,
              "status": "failure",
              "end": 1596865326792,
              "result": {
                "message": "handler.outputStream is not a function",
                "name": "TypeError",
                "stack": "TypeError: handler.outputStream is not a function\n    at apply (/root/xen-orchestra/packages/xo-server/src/xo-mixins/backups-ng/index.js:1848:35)\n    at iteratee (/root/xen-orchestra/node_modules/golike-defer/src/index.js:57:17)\n    at /root/xen-orchestra/@xen-orchestra/async-map/src/index.js:32:17\n    at Promise._execute (/root/xen-orchestra/node_modules/bluebird/js/release/debuggability.js:384:9)\n    at Promise._resolveFromExecutor (/root/xen-orchestra/node_modules/bluebird/js/release/promise.js:518:18)\n    at new Promise (/root/xen-orchestra/node_modules/bluebird/js/release/promise.js:103:10)\n    at /root/xen-orchestra/@xen-orchestra/async-map/src/index.js:31:7\n    at /root/xen-orchestra/node_modules/lodash/_baseMap.js:17:23\n    at /root/xen-orchestra/node_modules/lodash/_createBaseFor.js:17:11\n    at baseForOwn (/root/xen-orchestra/node_modules/lodash/_baseForOwn.js:13:20)\n    at /root/xen-orchestra/node_modules/lodash/_createBaseEach.js:17:14\n    at baseMap (/root/xen-orchestra/node_modules/lodash/_baseMap.js:16:3)\n    at map (/root/xen-orchestra/node_modules/lodash/map.js:50:10)\n    at asyncMap (/root/xen-orchestra/@xen-orchestra/async-map/src/index.js:30:5)\n    at /root/xen-orchestra/packages/xo-server/src/xo-mixins/backups-ng/index.js:1822:17\n    at /root/xen-orchestra/packages/xo-server/src/xo-mixins/backups-ng/index.js:369:21"
              }
            }
          ],
          "end": 1596865326792,
          "result": {
            "message": "handler.outputStream is not a function",
            "name": "TypeError",
            "stack": "TypeError: handler.outputStream is not a function\n    at apply (/root/xen-orchestra/packages/xo-server/src/xo-mixins/backups-ng/index.js:1848:35)\n    at iteratee (/root/xen-orchestra/node_modules/golike-defer/src/index.js:57:17)\n    at /root/xen-orchestra/@xen-orchestra/async-map/src/index.js:32:17\n    at Promise._execute (/root/xen-orchestra/node_modules/bluebird/js/release/debuggability.js:384:9)\n    at Promise._resolveFromExecutor (/root/xen-orchestra/node_modules/bluebird/js/release/promise.js:518:18)\n    at new Promise (/root/xen-orchestra/node_modules/bluebird/js/release/promise.js:103:10)\n    at /root/xen-orchestra/@xen-orchestra/async-map/src/index.js:31:7\n    at /root/xen-orchestra/node_modules/lodash/_baseMap.js:17:23\n    at /root/xen-orchestra/node_modules/lodash/_createBaseFor.js:17:11\n    at baseForOwn (/root/xen-orchestra/node_modules/lodash/_baseForOwn.js:13:20)\n    at /root/xen-orchestra/node_modules/lodash/_createBaseEach.js:17:14\n    at baseMap (/root/xen-orchestra/node_modules/lodash/_baseMap.js:16:3)\n    at map (/root/xen-orchestra/node_modules/lodash/map.js:50:10)\n    at asyncMap (/root/xen-orchestra/@xen-orchestra/async-map/src/index.js:30:5)\n    at /root/xen-orchestra/packages/xo-server/src/xo-mixins/backups-ng/index.js:1822:17\n    at /root/xen-orchestra/packages/xo-server/src/xo-mixins/backups-ng/index.js:369:21"
          }
        },
        {
          "id": "1596865326793",
          "message": "set snapshot.other_config[xo:backup:exported]",
          "start": 1596865326793,
          "status": "success",
          "end": 1596865326812
        }

Thanks!