OneSignal not working with AMP

As requested, this is exactly the same issue as described in: https://www.ads-software.com/support/topic/onesignal-push-notification-not-working-on-amp-pages/

Website is not proxied via Cloudflare. There is valid SSL. There is no cache plugins or anything activated.

The subscribe to notifications button is visible on AMP pages and once pressed, it directs you to grant the required permissions. Yet, the device doesn’t get added to the OneSignal subscribers list. Also to be noted, that the button only works once and afterwards becomes unresponsive.

Non-AMP version of the site works fine with OneSignal.