No meu caso não estava instalado o module rewrite:

  <rewrite>
        <rules>
          <rule name="Redirect to HTTPS" stopProcessing="true">
            <match url="(.*)" />
            <conditions>
              <add input="{HTTPS}" pattern="off" ignoreCase="true" />
            </conditions>
            <action type="Redirect" url="https://{HTTP_HOST}/{R:1}" redirectType="Permanent" />
          </rule>
        </rules>
      </rewrite>