The message The service is unavailable.
is actually IIS's default HTTP 503 message. The problem is that IIS (not nginx) intercepts and rewrites the response by default.
You can configure this behavior by using the httpErrors
element in your web.config
like so:
<system.webServer>
<httpErrors existingResponse="PassThrough"></httpErrors>
</system.webServer>
Note that configuring this may have security implications for your application. You can read more about configuring httpErrors
in this article