Based on a quick search there are a number of reasons why the public key maybe blank such as no SSL cert on the server in apache, some windows problems, some script installation problems from fantastico installs, some configuration problems relating to directories.
One interesting point is that a few ppl complained of SSO not working between Moodle and other systems. You may wish to try searching the forums in Moodle and google to see if any of them are related to your situation.
