Push-notification not working

RC release-0.67.0
Installed RC and read some of the discussions about configuring the push-service. For about two weeks it worked fine, then no push notifications arrived at the smartphones anymore. The server log was full of errors complaining " Error sending push to gateway (0 try) -> { Error: connect ETIMEDOUT" etc.
After a reboot it worked for a while, now the problem reappears.
While clicking on the test-button for push-notification it says something like: “No tokens for this user”. Could you please fix this annoying issue? Thanks.

Cheers, Hans