POST /api/v2/flows/executions returning 500 on valid flowId

Hit a weird 500 Internal Server Error when kicking off an Architect flow via the API. The flowId checks out, and the payload is minimal. No useful error body, just a generic 500. Tried it with curl and the TS SDK, same result. Here’s the call:

{
 "flowId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
 "parameters": {
 "input": "test"
 }
}

Anyone else seeing this?