Also, the actual error in the detailed report is in my case:

...
  "tasks": [
    {
      "data": {
        "type": "VM",
        "id": "74ed4e04-3bd9-1c68-f1e0-3c7a0fb567f7"
      },
      "id": "1748127565992",
      "message": "backup VM",
      "start": 1748127565992,
      "status": "failure",
      "end": 1748127565993,
      "result": {
        "call": {
          "duration": 91,
          "method": "session.login_with_password",
          "params": "* obfuscated *"
        },
        "message": "unexpect statusCode 302",
        "name": "Error",
        "stack": "Error: unexpect statusCode 302\n    at file:///opt/xen-orchestra/packages/xen-api/transports/json-rpc.mjs:24:13\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)"
      }
    },
...

@syscon-chrisl Did you find a solution for this?

I am asking because I ran into the very same error by switching my storage network. I tested a few different setups and found that I can modify the interface for all non-master nodes in my pool to the new interface without any issues; but as soon as I switch the master node, the backups will stop working while normal operation continues without any issues.

I also tried changing the pool master afterwards to a node that has the modified storage interface, but the error would then reappear.

It also doesn't seem to make a difference if I change the interface from XO (by setting the new interface to the same IP settings and then disabling the old interface and removing its IP) or through XCP-ng Center, the result seems to be same both times.

I set up a new XOA as well as a new XO (from sources) instance (and ran different versions of XO there) as I thought it was something with my current XO instance, but to no avail ...