We reply in German and English.
Questions, bugs, requests: support@fellbert.app. Please include which phone you use and which version of the app, it is shown at the bottom of the settings.
Data protection requests go to datenschutz@fellbert.app.
This is almost always down to the phone’s battery-saving settings. Allow Fellbert to run in the background and exclude the app from battery optimisation. On Android the app also needs permission for exact alarms.
This is handled by the store, not by us: on iPhone under Settings → your name → Subscriptions, on Android in the Play Store under Payments and subscriptions.