Hi Markus,
It sounds like the air is escaping through the zipper – a few small bubbles could add up to 1-2 breaths per hour, no? This could probably be resolved by cleaning the zipper and reapplying lubricant. If the zipper isn’t leaking and the seams are all sealed and the fabric isn’t damaged and there’s no dirt in the Boston valve or top-up valve, the packraft should hold air overnight.
Cheers!