Login problem with Filr-Apps on Filr 2023.4

I updated the Filr server to 2023.4. The web client works; the Windows client works.

The iOS app is only available in version 2023.2. When logging into the app, "Connection" appears, then an error: "Not a valid Filr System. Contact your Filr Administrator."

Similar with the Android 2023.4 app. Error message after login: "Error logging in. Connection not possible..." Access via the browser works perfectly on the same device.

What's going wrong here? What can I do?