I have now tested the plugin on a new multisite installation and everything, really everything, looked perfect.
Until this one point!
No matter whether I create a free product or a payment plan, as soon as I try to book it, "Payment service provider not registered ". And believe me, I have REALLY set everything up.
It does not work. You can't order a product or plan. At least with Free I would have expected that but even there it doesn't work.
What is the problem here? Is the problem known? Or am I really doing something fundamentally wrong?