After several restarts I was debugging the issue today and discovered that the configurationService in Jollyday was empty. So I restarted OpenHAB again to debug the initial loading of the services and… the error has vanished into thin air.
I appear to have this exact problem (on openHAB 4.3.2 Release Build). isBankHoliday is gives the error and isWeekend is working. However, I have restarted several times but in my case the problem remains. On 4.1 I did not have the problem. I skipped 4.2. I don’t think I have the skills to debug this myself.