400 Bad Request on all gws calendar +insert commands

Quite suddenly I was unable to use the gws calendar +insert command. It has worked reliably for months but today it suddenly stopped working and I constantly get:

{
  "error": {
    "code": 400,
    "message": "Bad Request",
    "reason": "badRequest"
  }
}
error[api]: Bad Request

I have tried re-authenticating, fresh install, but it seems the problem is server-side?