When I scan the generated QR code on the Wireguard Tunnel configuration page, I get "Page cannot be opened, server is not responding".

You are on the Wireguard Tunnel configuration page https://www.rhrk.uni-kl.de/vpnwg/ and have generated a QR code.

The generated QR code cannot be scanned using the camera app. Scan the QR code via the Wireguard app.
See the instructions below:
Apple iOS
Android