If your IIS Express server just "stops working". Error Code 403.14 Forbidden

I’ve recently decided to update a webservice of mine. I added some business code and also added some startup code in global.asax. The work took a couple of hours and this was the first time in a month I touched the project. When I hit F5 to debug the browser just said HTTP Error 403.14 - Forbidden. Well - what is this? No Yellow Screen Of Death or anything, just a bad IIS response.

403.14 Forbidden

I went through my code, tried to change ports on IIS Express and cleaned up my applicationhost.config. Nothing worked.

I started going back to older commits and everything worked. Turns out in the changes I made I introduced async/await in the global.asax Application_start and returned a Task instead of void.

Removed the async/await and everything worked again!

Hopefully someone will find this helpful while googling in despair!