Looks like an NPE here:
https://github.com/aerogear/aerogear-unifiedpush-server/blob/1.1.x-dev/service/src/main/java/org/jboss/aerogear/unifiedpush/service/metrics/PushMessageMetricsService.java#L159

Which is weird, as the push message ID is there, since entering the if block - also the variantID (and secret) is correct, otherwise a 401 would have been returned.

I wonder why this NPE does occur... hrm...

is the bug always reproducible ?

E.g. can you test with a new/different variant, and than send a new/different push message ?