Premium not available?

I have OneCalendar on a Fairphone 4 with e/OS. It works absolutely fine! But there is no option to upgrade to Premium. Under "options" -> "Kalenderfarben" it says: "Fehler beim Laden der Premium Produktinfo: Product not found". I attach a screenshot. What can I do to get the Premium version? I have done all updates and also de- and re-installed onecalendar.