My MVC app has, until a few minutes ago, been working fine (with asp/net membership as part of the solution). However, without knowingly changing anything relevant every req
Option-1:
In applicationhost.config check if there is any entry as shown below. If there is any such entry change anonymousAuthetication enabled value from 'false' to 'true'.
<location path="YOUR-APPLICATION-NAME">
<system.webServer>
<security>
<authentication>
<anonymousAuthentication enabled="false" />
</authentication>
</security>
</system.webServer>
</location>
Option-2:
If you are using visual studio, make sure that anonymousAuthentication is enabled.
This defeats the purpose -- if you enable anonymous auth, you are no longer using Active Directory... here is a better answer ....
HTTP Error 401.2 - Unauthorized for new site in a working app pool