I usually check 1) Permissions: does the app pool identify have requisite permissions on the directory; 2) Are the bindings correct for the site; and 3) default document at the root and/or site level default.aspx is the first default document specified; 4) Elon Musk insists the best configuration is no configuration if you have a configuration option specified is it necessary for the site to function if no remove that setting; 5) inspect web application logs in ~/portals/_default/logs; 6) are there any unhandled module errors which prevent the page from loading and redirect to the error page; 7) is the error page properly configured; 8) If you are using ActionForms check it's separate error log directory in the default logs directory.

Checking permissions, bindings, and default documents is an important part of the process, as is looking for any unhandled errors or issues with the error page configuration.

Ensure the alias you are using to access the portal is contained within the dbo.PortalAlias table. If it isn't add it and ensure it is not selected as default.

Ensure that you have set HttpErrors "Detailed" or "DetailedRemoteOnly" and CustomErrors "Off". This will cause asp.net to display any .NET code errors.

In addition to the steps you have listed, there are a few other things you can check to help resolve any issues you may be experiencing. For example, you can try disabling any custom modules or extensions that you have installed on the website, as these can sometimes cause conflicts or issues. You can also try clearing the cache and restarting the website to see if that resolves the problem.

Another thing to check is the server environment itself. Make sure that the server has enough resources, such as CPU and memory, to support the website. You can also try disabling any other applications or services that may be competing for resources with IIS and DotNetNuke.

Overall, the key to troubleshooting IIS and DotNetNuke is to carefully investigate all of the different factors that could be causing the issue. By following a systematic approach and ruling out potential causes one by one, you should be able to identify and resolve the problem.