Push-Proxy Error: device token has incorrect length

since we upgraded everything to 3.4 (TEAM Edition Server, iOS Apps and push-proxy), the Push-Notifications are not working anymore. We see the following error message at the push-proxy:

2016/10/14 11:08:37 INFO Sending apple push notification
2016/10/14 11:08:37 ERROR Failed to send apple push sid=xxxxxxxxxxxxxxxxxxxxxxxx did= err=device token has incorrect length

We have checked the installation several times, certificates are OK, too.
Does anyone have an idea where this error comes from? Could it be a problem within the iOS App?

Hi @jhr, thanks for the report, adding @jwilander as he might have some ideas on troubleshooting push notifications.