Office.context.mailbox.getCallbackTokenAsync() empty token - outlook-addin

Question

I'm having a problem with a web add-in for Outlook.
The Outlook client is 2016 (MSI) which I believe means that it supports no higher than JavaScript API 1.4, opposed to Outlook 2016 (C2R) which, as I recall it, supports JavaScript API 1.6 or maybe even higher.
Anyway, I'm trying to use the method Office.context.mailbox.getCallbackTokenAsync(asyncResult) which has previously worked just fine on the server where it is used, but now it has stopped working for some odd reason.
The asyncResult is now empty or rather the token is empty.
{"value":"","status":"succeeded"}
How can the token be empty all of the sudden when this add-in used to work perfectly?
According to the admin of the server, it has received Windows updates on the date that this stopped working for both Office and Outlook specifically.
The Outlook clients connect to an Exchange 2013 (CU7 December 9, 2014 : 15.0.1044.25) which has also received some updates.
Both servers have been rebooted since then, but nothing has changed. The token remains empty.
Can anyone shed some light on what could be the cause of this if anyone knows that is, because all I can really do myself at this point is guess?
UPDATE 1
I have now been given permission to install Fiddler and I have found the request and respond regarding the attempt to retrieve the token.
Can any of you who know the Exchange server inside out see what is going on here, because I don't see any reasoning as to what is failing, except that the response message indicates that the request is faulty somehow (which hasn't been changed for more then a year at least).
Here is the request (some names have been replaced with something generic).
REQUEST
POST https://<domain>/ews/Exchange.asmx HTTP/1.1
Cache-Control: no-cache
Connection: Keep-Alive
Pragma: no-cache
Content-Type: text/xml; charset=utf-8
User-Agent: Microsoft Office/16.0 (Windows NT 6.3; Microsoft Outlook 16.0.4849; Pro)
X-User-Identity: <account>#<domain>.com
Depth: 0
Content-Length: 801
Host: <host>
Authorization: Negotiate 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
Cookie: OutlookSession="{994C5944-A93C-4830-9E6F-605881790815}"; ClientId=PRHSVIWKYUDISQLQPQ
<?xml version="1.0"?>
<q:Envelope
xmlns:ex12t="http://schemas.microsoft.com/exchange/services/2006/types"
xmlns:ex12m="http://schemas.microsoft.com/exchange/services/2006/messages"
xmlns:q="http://schemas.xmlsoap.org/soap/envelope/">
<q:Header>
<ex12t:RequestServerVersion Version="Exchange2012"></ex12t:RequestServerVersion>
</q:Header>
<q:Body>
<ex12m:GetClientAccessToken>
<ex12m:TokenRequests>
<ex12t:TokenRequest>
<ex12t:Id>214c1212-e3ff-45eb-9218-2deb35d6b8b9</ex12t:Id>
<ex12t:TokenType>ScopedToken</ex12t:TokenType>
<ex12t:Scope>ParentItemId:AAMkADRiMzkyMjhmLWQ1NGItNDY0Mi04Nzk0LWYyNzMzZWQ2ZGE5MABGAAAAAAApHj7qoKF1QY4+pcwfu7uCBwCHPrayw2+bT5ByF4j5Y8QZAAAAAAEMAACHPrayw2+bT5ByF4j5Y8QZAAAAAAFxAAA=</ex12t:Scope>
</ex12t:TokenRequest>
</ex12m:TokenRequests>
</ex12m:GetClientAccessToken>
</q:Body>
</q:Envelope>
RESPONSE (some names have been replaced with something generic).
HTTP/1.1 200 OK
Cache-Control: private
Content-Type: text/xml; charset=utf-8
Server: Microsoft-IIS/8.5
request-id: 1a7cbf79-8ba3-4a73-bfa2-1733d841b2b1
X-CalculatedBETarget: <server>.local
X-DiagInfo: <server>
X-BEServer: <server>
X-AspNet-Version: 4.0.30319
Set-Cookie: exchangecookie=2cd797c5290345a7861dfe60e16ecc12; expires=Thu, 29-Oct-2020 10:21:15 GMT; path=/; HttpOnly
Set-Cookie: X-BackEndCookie=S-1-5-21-2060358956-2462126529-2132206371-1263=u56Lnp2ejJqBmpzHns+cypzSncaZzdLLmprH0p3HxsvSm5yaycuazMieys/MgYHNz87G0s7O0s3Hq87Pxc3Oxc7K; expires=Thu, 28-Nov-2019 09:21:15 GMT; path=/ews; secure; HttpOnly
Persistent-Auth: true
X-Powered-By: ASP.NET
X-FEServer: <server>
Date: Tue, 29 Oct 2019 10:21:15 GMT
Content-Length: 1148
<?xml version="1.0" encoding="utf-8"?>
<s:Envelope
xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Header>
<h:ServerVersionInfo MajorVersion="15" MinorVersion="0" MajorBuildNumber="1044" MinorBuildNumber="21" Version="V2_22"
xmlns:h="http://schemas.microsoft.com/exchange/services/2006/types"
xmlns="http://schemas.microsoft.com/exchange/services/2006/types"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
</s:Header>
<s:Body
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<m:GetClientAccessTokenResponse
xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages"
xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">
<m:ResponseMessages>
<m:GetClientAccessTokenResponseMessage ResponseClass="Error">
<m:MessageText>The token for this extension could not be retrieved.</m:MessageText>
<m:ResponseCode>ErrorInvalidClientAccessTokenRequest</m:ResponseCode>
<m:DescriptiveLinkKey>0</m:DescriptiveLinkKey>
</m:GetClientAccessTokenResponseMessage>
</m:ResponseMessages>
</m:GetClientAccessTokenResponse>
</s:Body>
</s:Envelope>