I'm getting similar too, I get the "entries must be sorted in asc order 2025-01 2025-52" error (see below) hope that's helpful.
Based on the comment about removing the Long-term retention of backups data, I just removed the Number of weekly backups kept entry and re-ran, this allowed the backup to run. I kept my Number of daily backups kept limit which I haven't reached yet.
      "message": "backup VM",
      "start": 1735640611472,
      "status": "failure",
      "tasks": [
        {
          "id": "1735640611748",
          "message": "snapshot",
          "start": 1735640611748,
          "status": "success",
          "end": 1735640616464,
          "result": "1f91db46-fbfe-ba09-9f8b-1c350d7493fd"
        },
        {
          "data": {
            "id": "59e1ac20-f03d-45b4-953d-6fb7277537c2",
            "type": "remote",
            "isFull": true
          },
          "id": "1735640616501",
          "message": "export",
          "start": 1735640616501,
          "status": "failure",
          "end": 1735640616505,
          "result": {
            "generatedMessage": false,
            "code": "ERR_ASSERTION",
            "actual": false,
            "expected": true,
            "operator": "strictEqual",
            "message": "entries must be sorted in asc order 2025-01 2025-52",
            "name": "AssertionError",
            "stack": "AssertionError [ERR_ASSERTION]: entries must be sorted in asc order 2025-01 2025-52\n    at getOldEntries (file:///home/root/xen-orchestra/@xen-orchestra/backups/_getOldEntries.mjs:116:18)\n    at FullRemoteWriter._run (file:///home/root/xen-orchestra/@xen-orchestra/backups/_runners/_writers/FullRemoteWriter.mjs:39:24)\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)\n    at async FullRemoteWriter.run (file:///home/root/xen-orchestra/@xen-orchestra/backups/_runners/_writers/_AbstractFullWriter.mjs:6:14)"
          }