Well, I tried removing the rds roles and adding them again. It brought me back to my original problem of receiving the Authentication Mode="Forms" error.
It is basically a default installation of RDS on server 2012 - I have not tried to change the authentication mode.
When I go to the url of the rds website (https://rds.(domain.com)RDWeb) which is not on the default site, I get the above mentioned error.
I get the same error whether I try from outside of the network, inside the network, on the server itself and on the server using the ip address.
Any help would be greatly appreciated.
Thanks!