ASP.NET Core 1.1 runs fine locally but when publishing to Azure says "An error occurred while starting the application."

Since many different problems can cause this error page, I can strongly recommend the following in order to determine the root cause quickly and easily, without wrestling Azure (or any server/platform for that matter) to get logs.

You can enable extremely helpful developer friendly error messages at startup by setting the .UseSetting("detailedErrors", "true") and .CaptureStartupErrors(true) actions in your Program.cs file.

For ASP.NET Core 1.x

public static void Main(string[] args)
{
  var host = new WebHostBuilder()
      .UseKestrel()
      .UseContentRoot(Directory.GetCurrentDirectory())
      .UseSetting("detailedErrors", "true")
      .UseIISIntegration()
      .UseStartup<Startup>()
      .CaptureStartupErrors(true)
      .Build();

  host.Run();
}

(2018/07) Update for ASP.NET Core 2.1

public class Program  
{
    public static void Main(string[] args)
    {
        BuildWebHost(args).Run();
    }

    public static IWebHost BuildWebHost(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .CaptureStartupErrors(true)
            .UseSetting("detailedErrors", "true")
            .UseStartup<Startup>()
            .Build();
}

• These settings should be removed as soon as your troubleshooting is complete so as not to expose your application to malicious attacks.

Connect via an sftp client and delete everything in the site/wwwroot folder manually. Republish

I have had nothing but problems since I migrated an application I have hosted on Azure to .net core from MVC 4.

At one point a few weeks ago I was unable to get a project to run after a successful publish. I even tried twice to delete the entire App Service profile and recreate it with the same name. However when I appended a '2' to the App Service name (to create a never before used app service) publishing the exact same project with 0 changes worked perfectly. What exactly does a delete do if I can publish successfully to a new app service but not a deleted and recreated one? Remove Existing Files At Destination was checked in each publish, that didn't do anything either.

I had the same error today as pictured in the OP in my #2 site. It occurred after attempting to update a number of asp nuget packages and re-deploy. Really not wanting to have to move on to iteration myApp3 of my app service, I decided to use the FTP information provided in the azure overview page. I navigated to Site/wwwroot and deleted everything inside from the FTP client. I then published the application, and it worked. I can only conclude that the 'Delete' checkbox doesn't work properly.

Thanks everyone for your suggestions. The only thing that worked in the end though is deleting that Azure web app that I couldn't publish to, and creating a brand new one. I guess maybe some of the .dlls from the previous runtime environment were still hanging around or not being updated... Whatever it was, re-creating it worked. Hopefully I don't get this error again though, because you can't really do this kind of stuff in production.

Making changes to the global.json file seemed to have no effect.

Creating an entirely new API from a template didn't help either, the issue was with the Azure Web App itself, as everything was running fine locally.

Another very helpful tip was to add logging (and the "logs" file in the root) as per the other answer. That at least pointed me in the right direction. Also checking your runtime with dotnet --version.

Again thanks for everyone's help!