Problems trying to integrate Shibboleth authentication to RT

I’m having trouble getting RT to work with shibboleth ( http://shibboleth.internet2.edu/
)

I’ve installed Shibboleth SP to the server, and am able to
authenticate with it.

I’ve also installed RT-Authen-Federation module ( http://search.cpan.org/dist/RT-Authen-Federation/
) which should enable us to login using shibboleth authentication,
but right now when I try to login to our RT, it first shows shibboleth
login screen. After which it prompts for the RT login, because I have
enabled WebFallbackToInternalAuth setting. After that I can login with
normal RT username/password.

I don’t get any errors or anything in to logs.

I have RT 3.6.1 installed on Debian etch with apache 2.2 + fastcgi and
shibboleth sp 2.1 installed from sources.

If anyone has any good hints where to look next, I would appreciate
them.