On my local box IIS just stopped working when the sites are mapped to IIS and not the DEV IIS Express instance.
After a bit of searching I did the following to my web.config. I have not had time to research what caused the change in my box that has broken it, but for now I need it to just work.
In the appSettings I added these two sections for my old ASP.NET Web Forms application
<add key="aspnet:UseTaskFriendlySynchronizationContext" value="false" /> <add key="ValidationSettings:UnobtrusiveValidationMode" value="None" />