I recently loaded up one of my servers and out of nowhere, I ran into this error:

Server Configuration Error The server has encountered a configuration error attempting to process your request. The configuration parameter MD_CUSTOM_ERROR (6008) has an invalid value. Please contact the server administrator for assistance.

I searched the web but couldn't find anything useful. I then started exploring the website's configuration under IIS Properties and I received a similar error when loading up the tab "Custom Errors." I saw a malformed error (401;3 was pointing to C:\windows\help for some reason), set it to default and the site worked again.