Push notifications not delivered

I’m unable to receive push notifications. When I run ./occ notification:test-push, I got this suspicuous message:

Signed encrypted push subject
Push notification sent but response was not parsable, using an outdated push proxy? [https://push-notifications.nextcloud.com]: no reason given

The notification did not arrive tho. Why it might happen? Broken tokens? Cleanup needed?

I see that the database has tokens for the registered devices, but maybe those tokens got outdated or broken somehow?