SonarQube IIS reverse proxy config -- USE THE OTHER web.config template please. It includes changes to the request size.

web.config_old_wo_size_fix

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <rewrite>
            <outboundRules>
                <rule name="ReverseProxyOutboundRule1" preCondition="IsRedirection">
                    <match filterByTags="A, Form, Img" serverVariable="RESPONSE_LOCATION"
                           pattern="^http://[^/]+/(.*)" />
                    <action type="Rewrite" value="https://PUBLIC_SONARQUBE_URL/{R:1}" />
                </rule>
                <preConditions>
                    <preCondition name="IsRedirection">
                        <add input="{RESPONSE_STATUS}" pattern="3\d\d" />
                    </preCondition>
                    <preCondition name="ResponseIsHtml1">
                        <add input="{RESPONSE_CONTENT_TYPE}" pattern="^text/html" />
                    </preCondition>
                </preConditions>
            </outboundRules>
            <rules>
                <rule name="ReverseProxyInboundRule1" stopProcessing="true">
                    <match url="(.*)" />
                    <action type="Rewrite" url="http://PRIVATE_SONAR_URL:9000/{R:1}" />
                    <serverVariables>
                        <set name="X_FORWARDED_PROTO" value="https" />
                        <set name="ORIGINAL_URL" value="{HTTP_HOST}" />
                    </serverVariables>
                </rule>
            </rules>
        </rewrite>
    </system.webServer>
</configuration>