PushManager subscription – AbortError: DOMException: Error retrieving push-subscription – Firefox for Android

On Firefox for Android / Fennec (newest version) I cannot subscribe to push-notifications.
You can check it out on cleverpush.com.
It’s the same problem as mine.

My script subscribe.js should do the following:

1. Get permissions for sending Notifications – works fine

2. check for an existing subscription of this browser / device (ServiceWorkerRegistration.pushManager.getSubscription();) – this works fine on Safari, Chrome and Firefox for Windows, Linux and macOS. It fails on Firefox for Android / Fennec (F-Droid).
   The error is an AbortError: DOMException: Could not retrieve push subscription.. The same error happens on cleverpush.com with Firefox for Android / Fennec.

3. When the user clicks a Button “Register for Push-Notifications”, it executes const subscription = await ServiceWorkerRegistration.pushManager.subscribe(...);. Same error like in (2.)

Many thanks if someone can tell me “this is a bug from mozilla” or “you made some mistake, here the explanation” or “this question has already answers here: …”!