It seems your Web Application Authentication settings got corrupted somehow. You can try to clean this up by simply Enabling and Disabling Anonymous Access.

It seems your Web Application Authentication settings got corrupted somehow. You can try to clean this up by simply Enabling and Disabling Anonymous Access.

You can do this by

1. Open Central Admin Site
2. Open Application Management & Select Manage Web Applications
3. Select web application in question and open Authentication Providers
4. Choose default authentication provider zone
5. Check Enable Anonymous Access and Save

Now do the same steps as above and in step#5 - Uncheck Enable Anonymous Access