"VDI_MISSING" error message upon trying to boot a UEFI VM...

[Station-Already]

Hey all -

I'm trying to setup a UEFI-based Windows Server 2019 Standard x64 server to test Remote Desktop Services on a newer platform (then we needn't worry about the best Guest Tools for Windows Server 2008 R2!), but I get an error message, VDI_MISSING . I don't understand why, I've created VMs like this before, and it's worked fine. I even tried deleting the associated 80 GB disk and recreating it, but nope - same error. I don't get it.

Worth mentioning: I tried this with both the latest stable (5.64.0) and a recent... er, latest (5.65.1).

vm.start
{
 "id": "50516f0f-8459-8548-5ee3-42197e438f4d",
 "bypassMacAddressesCheck": false,
 "force": false
}
{
 "code": "VDI_MISSING",
 "params": [
   "OpaqueRef:4510e18f-14fb-4003-b4c7-4081bd6d1e6d",
   "OpaqueRef:8f2ea38e-6bef-41f4-9f85-73e02ee44ec6"
 ],
 "call": {
   "method": "VM.start",
   "params": [
     "OpaqueRef:b4f01459-4414-40ff-8f91-b742f3cfe4e9",
     false,
     false
   ]
 },
 "message": "VDI_MISSING(OpaqueRef:4510e18f-14fb-4003-b4c7-4081bd6d1e6d, OpaqueRef:8f2ea38e-6bef-41f4-9f85-73e02ee44ec6)",
 "name": "XapiError",
 "stack": "XapiError: VDI_MISSING(OpaqueRef:4510e18f-14fb-4003-b4c7-4081bd6d1e6d, OpaqueRef:8f2ea38e-6bef-41f4-9f85-73e02ee44ec6)
   at Function.wrap (/usr/local/lib/node_modules/xo-server/node_modules/xen-api/src/_XapiError.js:16:12)
   at /usr/local/lib/node_modules/xo-server/node_modules/xen-api/src/transports/json-rpc.js:41:27
   at AsyncResource.runInAsyncScope (async_hooks.js:197:9)
   at cb (/usr/local/lib/node_modules/xo-server/node_modules/bluebird/js/release/util.js:355:42)
   at tryCatcher (/usr/local/lib/node_modules/xo-server/node_modules/bluebird/js/release/util.js:16:23)
   at Promise._settlePromiseFromHandler (/usr/local/lib/node_modules/xo-server/node_modules/bluebird/js/release/promise.js:547:31)
   at Promise._settlePromise (/usr/local/lib/node_modules/xo-server/node_modules/bluebird/js/release/promise.js:604:18)
   at Promise._settlePromise0 (/usr/local/lib/node_modules/xo-server/node_modules/bluebird/js/release/promise.js:649:10)
   at Promise._settlePromises (/usr/local/lib/node_modules/xo-server/node_modules/bluebird/js/release/promise.js:729:18)
   at _drainQueueStep (/usr/local/lib/node_modules/xo-server/node_modules/bluebird/js/release/async.js:93:12)
   at _drainQueue (/usr/local/lib/node_modules/xo-server/node_modules/bluebird/js/release/async.js:86:9)
   at Async._drainQueues (/usr/local/lib/node_modules/xo-server/node_modules/bluebird/js/release/async.js:102:5)
   at Immediate.Async.drainQueues [as _onImmediate] (/usr/local/lib/node_modules/xo-server/node_modules/bluebird/js/release/async.js:15:14)
   at processImmediate (internal/timers.js:464:21)
   at process.topLevelDomainCallback (domain.js:147:15)
   at process.callbackTrampoline (internal/async_hooks.js:129:24)"
}