Lesefluss needs Android System WebView 111 or newer. This device has version unknown.
Update Android System WebView in the Play Store, then open Lesefluss again.
No Play Store on this device? The WebView usually updates with the device maker's system updates.
Help: lesefluss.app/docs?tab=troubleshooting
Open Lesefluss in a current version of Chrome, Firefox or Safari, or use the Android app.