This is a weird one. We periodically get reports from clients on their Umbraco v10 sites that they are getting this error message when trying to publish a page. It only seems to happen in Production, so I can never reproduce locally, but I've managed to see the error in the Network tab: The client gets a red notification banner saying: "One or more errors occurred. (Value cannot be null. (Parameter 'key'))" This is being caused by

/umbraco/backoffice/umbracoapi/content/PostSave

returning a 500 error with the JSON response:

{
  "ExceptionMessage": "One or more errors occurred. (Value cannot be null. (Parameter 'key'))",
  "ExceptionType": null,
  "StackTrace": null
}

The thing is, an exception isn't actually logged in the trace log, so there's no way of knowing what actually is causing the error. Also, as far as I can tell, the page is actually published afterward. The error seems to indicate some kind of Dictionary type thing with a missing key? Anyone any idea what might be the cause, as without a stack trace or the ability to hook in a debugger, I'm at a loss. It also seems to happen randomly, but after an app restart will "fix" itself.