Some times after installing and configuring Async Server, the download and upload service URL becomes inaccessible and it gives Error 503: Service Unavailable while opening the URL. This mostly happens if you change the App Pool User and re-configure your Async Server.
If you check your Async server site on IIS manager’s application pool, it will appear in “Stopped” status.
If you try to start it again and access the service URL now, as soon as you open the URL, the site will again get stopped.
To resolve this issue; Right click your site in application pool, Go to Advanced settings and re-enter the identity credentials. Now restart the site and IIS and try again. You will now be able to access your service URL.