"Unable to launch the IIS Express Web server" error

If using VS2015, close the solution, delete the following file, then re-open the solution.

<<path_to_solution_folder>>\.vs\config\applicationhost.config

note the .vs folder may be hidden

The one thing that fixed this for me was using the following line in the <bindings> section for my site in the applicationhost.config file:

<bindings>
    <binding protocol="http" bindingInformation="*:8099:" />
</bindings>

The key was to simply remove localhost. Don't replace it with an asterisk, don't replace it with an IP or a computer name. Just leave it blank after the colon.

After doing this, I don't need to run Visual Studio as administrator, and I can freely change the Project Url in the project properties to the local IP or computer name. I then set up port forwarding and it was accessible to the Internet.

EDIT:

I've discovered one more quirk that is important to getting IIS Express to properly serve external requests.

1. If you are running Visual Studio/IIS Express as an administrator, you must not add a reservation to HTTP.SYS using the "netsh http add urlacl ..." command. Doing so will cause an HTTP 503 Service Unavailable error. Delete any reservations you've made in the URLACL to fix this.

2. If you are not running Visual Studio/IIS Express as an administrator, you must add a reservation to the URLACL.

I had exactly the same issue. Tried everything but finally one trick worked.

1. Delete folder IISExpress from "My Documents"
2. Load your project again, it will create IISExpress folder again with updated configuration
3. Check IISExpress folder has sufficient permissions to read-write the configuration file
4. Load project again and Run IISExpress. It should work.