I think the best first approach is to make sure to turn on detailed error messages via your web.config file, like this:
<configuration>
<system.webServer>
<httpErrors errorMode="Detailed"></httpErrors>
</system.webServer>
</configuration>
After doing this, you should get a more detailed error message from the server.
In my particular case, the more detailed error pointed out that my <defaultDocument> section of the web.config file was not allowed at the folder level where I'd placed my web.config. It said
This configuration section cannot be used at this path. This happens when the section is locked at a parent level. Locking is either by default (overrideModeDefault="Deny"), or set explicitly by a location tag with overrideMode="Deny" or the legacy allowOverride="false". "