ACS Pushes to iOs end with Sending status: Fail

Hello

We have ACS Push notifications feature in our application. It is almost working but while testing on iOS we have noticed that some pushes are not delivered. What is strange, when we try send a few pushes, we get pretty random sending statuses, like: success, success, fail, success, fail, success, success

I can't find anything weird in the logs except that mysterious "Sending status: Fail" so it's pretty hard to debug where the problem is.

Example details from the failing request:

Data sent to ACS cloud:

Payload sent to ACS:

{"channel":"public","payload":"{\"icon\":\"appicon\",\"sound\":\"default\",\"vibrate\":true,\"alert\":\"Your friend Ralf **** published the image  ****\",\"url\":\"http://beta.xxapplication.com/#/images/537f46dde4b079607358372a?ti=true\",\"ts\":\"be0d13a\",\"t\":\"2\",\"o\":\"537f46dde4b079607358372a\",\"badge\":\"+1\"}","to_ids":"537e77b32c2e99083b097aa3","version":"v1","session_id":"rQU-CmF9sS8NZK4tCM_guUTV3Fk","format":"json","key":"********","action":"notify","controller":"push_notification"}

Payload sent from ACS to iPhone device:

{"icon":"appicon","sound":"default","vibrate":true,"alert":"Your friend Ralf **** published the image ****","url":"http://beta.xxapplication.com/#/images/537f46dde4b079607358372a?ti=true","ts":"be0d13a","t":"2","o":"********","badge":"+1”}

Any ideas what could be wrong or how could we debug it more thoroughly? Any help really appreciated :)