Getting an EWS Failure error - unable to obtain callback token

I know this is similar to other issues, but I didn’t see any resolution on them. I’m the Exchange admin. New 2016 install. No proxy between workstation and server. Same error from Outlook for Windows and OWA.

Requirement set = 1.6
API used = EWS
contentLanguage = en-US
displayLanguage = en-US
hostname = Outlook
hostVersion = 16.0.11425.20204
itemId = true
itemType = message
itemClass = IPM.Note
permissions = 3
ERROR: Unable to obtain callback token.
Fallback to EWS.
{
  "value": null,
  "status": "failed",
  "error": {
    "name": "InternalServerError",
    "message": "The Exchange server returned an error. Please look at the diagnostics object for more information.",
    "code": 9042
  },
  "diagnostics": {
    "ErrorText": "The token for this extension could not be retrieved."
  }
}
Anonymous function()@https://mha.azurewebsites.net/Scripts/GetHeadersRest.js:37:17
Anonymous function()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:183743
Anonymous function()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:175387
agaveResponseCallback():1:6425
window.agaveHostCallback():1:8327
ERROR: Async Response
{
  "value": "",
  "status": "succeeded"
}
callbackEws()@https://mha.azurewebsites.net/Scripts/GetHeadersEWS.js:80:17
Anonymous function()@https://mha.azurewebsites.net/Scripts/GetHeadersEWS.js:47:13
t.onreadystatechange()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:189638
r.EwsRequest.prototype._callOnReadyStateChangeCallback$i$1()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:307185
Anonymous function()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:315041
ERROR: Original Response
{
  "wasProxySuccessful": true,
  "errorMessage": null,
  "body": "",
  "error": false
}
callbackEws()@https://mha.azurewebsites.net/Scripts/GetHeadersEWS.js:84:17
Anonymous function()@https://mha.azurewebsites.net/Scripts/GetHeadersEWS.js:47:13
t.onreadystatechange()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:189638
r.EwsRequest.prototype._callOnReadyStateChangeCallback$i$1()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:307185
Anonymous function()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:315041
ERROR: EWS callback failed : Failed to retrieve headers.
callbackEws()@https://mha.azurewebsites.net/Scripts/GetHeadersEWS.js:75:17
Anonymous function()@https://mha.azurewebsites.net/Scripts/GetHeadersEWS.js:47:13
t.onreadystatechange()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:189638
r.EwsRequest.prototype._callOnReadyStateChangeCallback$i$1()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:307185
Anonymous function()@https://appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:15:315041```