Wordpress behind ARR proxy - JSON request not signed - wordpress

Question

Using IIS ARR I have a reverse proxy to a WP blog hosted elsewhere on a Linux server.
These are the proxy rules:
<rewrite>
<rules>
<rule name="Redirect to HTTPS" stopProcessing="true">
<match url="(.*)" />
<conditions>
<add input="{HTTPS}" pattern="^OFF$" />
</conditions>
<action type="Redirect" url="https://{HTTP_HOST}/blog/{R:1}" redirectType="Permanent" />
</rule>
<rule name="Reverse proxy to blog" stopProcessing="true">
<match url="^(.*)" />
<serverVariables>
<set name="HTTP_X_ORIGINAL_ACCEPT_ENCODING" value="{HTTP_ACCEPT_ENCODING}" />
<set name="HTTP_ACCEPT_ENCODING" value="" />
</serverVariables>
<action type="Rewrite" url="http://111.22.33.444/blog/{R:1}" />
</rule>
</rules>
<outboundRules>
<rule name="Add application prefix" preCondition="IsHTML" stopProcessing="true">
<match filterByTags="A, Form, Img, Link, Script" pattern="^http://111.22.33.444/blog/(.*)" />
<action type="Rewrite" value="https://www.example.com/blog/{R:1}" />
</rule>
<rule name="RestoreAcceptEncoding" preCondition="NeedsRestoringAcceptEncoding">
<match serverVariable="HTTP_ACCEPT_ENCODING" pattern="^(.*)" />
<action type="Rewrite" value="{HTTP_X_ORIGINAL_ACCEPT_ENCODING}" />
</rule>
<preConditions>
<preCondition name="NeedsRestoringAcceptEncoding">
<add input="{HTTP_X_ORIGINAL_ACCEPT_ENCODING}" pattern=".+" />
</preCondition>
<preCondition name="IsHTML">
<add input="{HTTP_HOST}" pattern="111.22.33.444" />
<add input="{RESPONSE_CONTENT_TYPE}" pattern="^text/html" />
<add input="{URL}" negate="true" pattern="wp-admin/" />
</preCondition>
</preConditions>
</outboundRules>
</rewrite>
"wp-admin/" has been excluded from the IsHTML pre-condition as this was causing redirect loops in some areas of the admin.
The plugin Jetpack has been installed but this is failing to connect to Wordpress.com. Wordpress.com shows the error "There was an error retrieving your site settings." Looking at the errors in Plesk where the blog is actually hosted we see the URL below failed with a 400 bad request.
blog/?rest_route=%2Fjetpack.................
If this URL is called manually in a web browser, it gives the message "The request is not signed correctly".
Can anyone offer any advise on resolving this bad request issue please?