Internal Server Error on Hetzner Storageshare > Editing whiteboards not possible

I just installed the whiteboard app on a freshly installed Hetzner storageshare instance but I have trouble getting the whiteboard app to work. As expected I get the WebSocket warning…

But since I don't want to collaborate in real-time and just want to use the basic functionality without a WebSocket server opening whiteboards alone should work:

The websocket server is only needed for live collaboration - basic whiteboard functionality works without it.

As a first test I created two Whiteboards from the talk app (empty board & kanban board from the templates), but when I click on it, only the empty preview is shown, I am not able to do edit the Whiteboard at all. The exact same is happening while opening whiteboard files directly from the Files App > no editing tools (Creating Shapes, Text, etc.) appear:

So I thought, maybe there's still configuration needed. But when trying to access the Whiteboard Settings under the Admin Panel I get this error and no settings are :

Do other Storageshare users have the same problem? How to fix?

Start by reproducing the failure when opening a whiteboard from Talk or Files, then inspect the Internal Server Error from Whiteboard Settings in the Admin Panel and the WebSocket warning. Compare the behavior with the documented basic functionality without WebSockets; done means whiteboards show editing tools and the settings page loads on Hetzner Storageshare.