I'm terribly sorry. Once again the problem proved to be the human (me) instead of the code... I still had the zeo-client option set to True.
I think it's a bit tricky that the code triggered by setting the zeo-client option to True, depends on not having the rel-storage option set. This relation could perhaps be made a bit more explicit so the error message is more helpful.
But again, I wasn't paying attention so I should not blame the recipe for it.