Outside link to Class Search

In our Peoplesoft 8.56.08, we came up with a link that goes directly to the Class Search component without logging in, using a guest ID. It was working all right, but now if a user clicks on the link, they get the message "No default homepage tab found (219,7438)." In the message catalog, there's the additional explanation "You do not have access to any homepage tab Please contact your portal administrator." However, if they click the back button and try again, the page comes up correctly. If they reload, however, the error message stays.

Obviously, the guest user has access and the link itself does work, but not until it's primed or something by the first attempt. Does anyone else have experience with something like this or some suspicion how this is happening?