How to resolve the error 'The service is unavailable' when trying to upgrade Umbraco

My scenario

I was just doing an upgrade of this Umbraco website from 8.4.0 to 8.6.4, everything went smoothly locally, but when it came to doing the upgrade on the live site, I saw a message like this:

The service is unavailable.

The url was as expected

https://codeshare.co.uk/install/?redir=true&url=https%3a%2f%2fcodeshare.co.uk%2f

But for some reason I was getting this weird error and I didn't know how to get past it.

It had to be something to do with the configuration I had on the production site that wasn't on the local one.

I did some googling and I found an issue in the Umbraco GitHub repo which seemed to describe my problem and I found a good solution in the comments.

So for anyone else who is searching for this now or myself searching for it again in 6 months, here was the code which solved it:

The solution

I just needed to add this line to the umbraco location path config setting in the main web.config file.

<httpErrors errorMode="Custom" existingResponse="PassThrough" />

Here it is with the rest of the code that is already in the file:

<location path="umbraco">
    <system.webServer>
        <urlCompression doStaticCompression="false" doDynamicCompression="false" dynamicCompressionBeforeCache="false" />
        <httpErrors errorMode="Custom" existingResponse="PassThrough" />
    </system.webServer>
</location>

The solution was to let the errors pass through. Who knew?

You can also do it for the install path:

<location path="install">
    <system.webServer>
        <urlCompression doStaticCompression="false" doDynamicCompression="false" dynamicCompressionBeforeCache="false" />
        <httpErrors errorMode="Custom" existingResponse="PassThrough" />
    </system.webServer>
</location>

Anyway I hope this helped you out in your hour of need.