When using .NET Framework 4.5 and changing Sitecore configuration to use the "Forms" authentication mode instead of "None", Sitecore may not be able to properly log in users to the Sitecore Client. When trying to log in with valid credentials, the login page will be refreshed and the user will not be logged in.
This behavior is caused by code changes in .NET Framework 4.5.
More information about configuring authentication mode can be found at http://msdn.microsoft.com/en-us/library/aa291347(VS.71).aspx.
Configure Sitecore application pool to use .NET Framework 2.0 or install .NET 4.0 version instead of .NET 4.5.
Set the authentication mode to the default "None" value in the /Web.config file:
Add the following code to the /Global.asax file:
public void FormsAuthentication_OnAuthenticate(object sender, FormsAuthenticationEventArgs args)
args.User = Sitecore.Context.User;