Backup: ERR_OUT_OF_RANGE in RemoteVhdDisk.mergeBlock

[wralb]

Context: Home Lab running XCP_NG since 2018. Current H/W is 2x MS-A2 not pooled together; local storage. Backup to local Synology NAS.
XO: Manually built from sources (no 3rd part script), current commit 561e2

Problem: One of 12 VMs in a nightly backup set is suddenly throwing this error (full log pasted at end):

"code": "ERR_OUT_OF_RANGE",
"The value of \"offset\" is out of range. It must be >= 0 and <= 106492. Received 106496",

This has happened the last three nights (so with commit 561e2 and with a commit from a few days ago).

The one different thing about this VM is that it also has 4 hourly rolling snapshots retaining 7 of them.

I'm not sure where to look to debug this. XO has been restarted. I haven't tried restarting the toolstack or host, or deleting all the snapshots, just in case the issue vanishes and I lose the opportunity to debug.

Another observation is that the email report says there is a failure, but actually claims that all phases of the backup succeeded.

    {
      "data": {
        "type": "VM",
        "id": "c54a256e-0493-5c6d-a0cd-c3a4618f99a1",
        "name_label": "MailCW"
      },
      "id": "1772596867673",
      "message": "backup VM",
      "start": 1772596867673,
      "status": "failure",
      "tasks": [
        {
          "id": "1772596867683",
          "message": "clean-vm",
          "start": 1772596867683,
          "status": "failure",
          "tasks": [
            {
              "id": "1772596868690",
              "message": "merge",
              "start": 1772596868690,
              "status": "failure",
              "end": 1772596871939,
              "result": {
                "code": "ERR_OUT_OF_RANGE",
                "message": "The value of \"offset\" is out of range. It must be >= 0 and <= 106492. Received 106496",
                "name": "RangeError",
                "stack": "RangeError [ERR_OUT_OF_RANGE]: The value of \"offset\" is out of range. It must be >= 0 and <= 106492. Received 106496\n    at boundsError (node:internal/buffer:88:9)\n    at checkBounds (node:internal/buffer:57:5)\n    at checkInt (node:internal/buffer:76:3)\n    at writeU_Int32BE (node:internal/buffer:804:3)\n    at Buffer.writeUInt32BE (node:internal/buffer:817:10)\n    at VhdFile._setBatEntry (/opt/xen-orchestra/packages/vhd-lib/Vhd/VhdFile.js:308:16)\n    at VhdFile._createBlock (/opt/xen-orchestra/packages/vhd-lib/Vhd/VhdFile.js:321:16)\n    at VhdFile.writeEntireBlock (/opt/xen-orchestra/packages/vhd-lib/Vhd/VhdFile.js:344:30)\n    at RemoteVhdDisk.writeBlock (file:///opt/xen-orchestra/@xen-orchestra/backups/disks/RemoteVhdDisk.mjs:237:21)\n    at RemoteVhdDisk.mergeBlock (file:///opt/xen-orchestra/@xen-orchestra/backups/disks/RemoteVhdDisk.mjs:298:19)"
              }
            }
          ],
          "end": 1772596871939,
          "result": {
            "code": "ERR_OUT_OF_RANGE",
            "message": "The value of \"offset\" is out of range. It must be >= 0 and <= 106492. Received 106496",
            "name": "RangeError",
            "stack": "RangeError [ERR_OUT_OF_RANGE]: The value of \"offset\" is out of range. It must be >= 0 and <= 106492. Received 106496\n    at boundsError (node:internal/buffer:88:9)\n    at checkBounds (node:internal/buffer:57:5)\n    at checkInt (node:internal/buffer:76:3)\n    at writeU_Int32BE (node:internal/buffer:804:3)\n    at Buffer.writeUInt32BE (node:internal/buffer:817:10)\n    at VhdFile._setBatEntry (/opt/xen-orchestra/packages/vhd-lib/Vhd/VhdFile.js:308:16)\n    at VhdFile._createBlock (/opt/xen-orchestra/packages/vhd-lib/Vhd/VhdFile.js:321:16)\n    at VhdFile.writeEntireBlock (/opt/xen-orchestra/packages/vhd-lib/Vhd/VhdFile.js:344:30)\n    at RemoteVhdDisk.writeBlock (file:///opt/xen-orchestra/@xen-orchestra/backups/disks/RemoteVhdDisk.mjs:237:21)\n    at RemoteVhdDisk.mergeBlock (file:///opt/xen-orchestra/@xen-orchestra/backups/disks/RemoteVhdDisk.mjs:298:19)"
          }
        },
        {
          "id": "1772596872185",
          "message": "snapshot",
          "start": 1772596872185,
          "status": "success",
          "end": 1772596873726,
          "result": "04b94e3c-8443-84a0-5ac2-486d4e13ec93"
        },
        {
          "data": {
            "id": "fd6bbd08-52dd-43eb-84df-f93b54fc28e9",
            "isFull": false,
            "type": "remote"
          },
          "id": "1772596873726:0",
          "message": "export",
          "start": 1772596873726,
          "status": "success",
          "tasks": [
            {
              "id": "1772596875363",
              "message": "transfer",
              "start": 1772596875363,
              "status": "success",
              "end": 1772597059519,
              "result": {
                "size": 9678356480
              }
            },
            {
              "id": "1772597061803",
              "message": "clean-vm",
              "start": 1772597061803,
              "status": "success",
              "end": 1772597062717,
              "result": {
                "merge": true
              }
            }
          ],
          "end": 1772597062918
        }
      ],
      "end": 1772597062919
    },