Forms authentication fails after installing IIS 7.5

I recently had a customer that just moved from Windows 2008 to Windows 2008 R2 (changing from IIS 7 to IIS 7.5) and after the move they got some weird forms authentication errors in the event log
The problem
They have two applications (MyWebApp and MyLoginWebApp) where forms authentication is dealt with in MyLoginWebApp
<authentication mode="Forms">   <forms name="MyAppAuth" loginUrl="/MyLoginWebApp/Default.aspx" protection="All" timeout="720" path="/" slidingExpiration="true" /> </authentication>
Once the user is authenticated they are redirected back to MyWebApp but on the redirect, forms authentication fails (and logs events like the below), and redirects back to the MyLoginWebApp application.
Event ID: 1315 Event message: Forms authentication failed for the request Reason: The ticket supplied was invalid
The two applica ...