Recently when deploying a site to IIS I got the following error.
I tried several troubleshooting options such as checking the IIS mapping configuration to see if the .aspx pages were an accepted page extension, folder permissions on the deployment folder, and I knew for a fact that .net was correctly installed on my server.
After some more searching and trial and error we narrowed the problem down to the CustomError tag in the applications web.config file. The mode attribute was set to “Off” and therefore when the application threw an error, in my case an unauthorized user signed in, it didn’t know what to do with the redirected webpage.
Once I set the Mode to “On” and followed the rest of the redirect settings my site came up and the issue was resolved.