Sidecar only shows a black screen on iPad until reboot

It is 100% fixable by rebooting my Mac. It is also 100% annoying when I have to do that. After the reboot I can use Sidecar wirelessly very reliably. Until I disconnect. Or change any Sidecar setting (Like: Show touch bar on/off). After that there is a good chance I have to reboot my Mac for it to work again. 1. Why does it do that? 2. How do I temporarily fix that? (Say: Is there a terminal command?) 3. How do I permanently fix that?