There's a neat feature in ADFS in the web.config that displays exceptions on the error page which I find a MAJOR help when I'm configuring stuff.
Uncomment this piece:
!-- Display the exception message on the error page. Uncomment this, or add the key below to your
app settings if you want to see the exception message. The exception messages are localized in the
language of the server.--
The web.config you need is here:
So I changed the code in FormsSignIn to use an exception called exp instead of ex resulting in the error below.