At SpecialOpenIDServer.body.php line 192, the code is trying to access a "mode" property on an Auth_OpenID_UntrustedReturnURL object. This property does not exist, although looking at the posted request the value 'checkid_setup' is probably what is intended.
Due to the missing value, it misses the proper case in the switch and we wind up with an empty page returned to the user. When I correct for this, it gives me a form asking if I want to trust something-or-other which seems to be the intended behavior.
On the other hand, that trust form doesn't actually work when submitted; various errors about "The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "Auth_OpenID_UntrustedReturnURL" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition" get logged and eventually something fatals.