SSO not working with.NET 4.5.1 WebForms using Forms Authentication with custom Membership Provider

I have two programs using ASP.NET WebForms which I have upgraded to .NET 4.5.1. Both use Forms Authentication with a custom MembershipProvider class library that also targets 4.5.1. My problem is that single sign-on no longer works.
I have the shared machineKey in Windows\Microsoft.NET\Framework64\v4.0.30319\Config\web.config:
<machineKey decryption="AES" validationKey="[…]" decryptionKey="[…]" validation="SHA1" />
Also tried putting it in each program’s web.config.
I have tried adding the compatibilityMode property to the machineKey and various additions to web.config appSettings without success:
<add key="PreserveLoginUrl" value="true" />
<add key="enableSimpleMembership" value="false" />
<add key="autoFormsAuthentication" value="false" />
<add key="aspnet:UseLegacyEncryption" valu ...