IIS rewrite rule not working in live environment

Your rule should be like that:

<rule name="http to https" stopProcessing="true">    <match url=".*" />    <conditions>        <add input="{HTTPS}" pattern="^OFF$" />        <add input="{REQUEST_URI}" pattern="/backoffice" negate="true" />    </conditions>    <action type="Redirect" url="https://www.WEBSITENAME.com{R:0}" /></rule>

This rule will exclude requests with /backoffice path.

Also for issue of mixing content you need to fix your paths for css/js/images to relatives. Example:

<img src="/path/to/your/image.jpg"/>

Another way to fix mixed content is create outbound rule, which will change your output HTML (replace http: to https:):

<rewrite>  ...  <outboundRules>    <rule name="Rewrite external references to use HTTPS" preCondition="IsHTML">      <match filterByTags="Script, Link, Img, CustomTags" customTags="HTML5Tags" pattern="^http://(.*)$" />      <action type="Rewrite" value="https://{R:1}" />    </rule>    <preConditions>      <preCondition name="IsHTML">        <add input="{RESPONSE_CONTENT_TYPE}" pattern="^text/html" />      </preCondition>    </preConditions>    <customTags>      <tags name="HTML5Tags">        <tag name="Video" attribute="src" />      </tags>    </customTags>  </outboundRules></rewrite>

Using the previous answer as a starting point, i made a few minor changes, to use HTTP_HOST rather than REQUEST_URI for the pattern negation and it works.

<system.webServer>    <rewrite xdt:Transform="InsertIfMissing">        <rules>            <rule name="http to https" stopProcessing="true">                <match url=".*" />                <conditions>                    <add input="{HTTPS}" pattern="^OFF$" />                    <add input="{HTTP_HOST}" pattern="^backoffice\.WEBSITENAME\.com$" negate="true" />                </conditions>                <action type="Redirect" url="https://www.WEBSITENAME.com/{R:0}" />            </rule>        </rules>    </rewrite></system.webServer>