Hi Benjamin,
Thank you for the answer.
Max has posted this February that Poweramp Unlocker might me temporarily unavailable in Google play since he was moving his company to another country.. I just thought that 6 months seem to be too long for a "temporary" problem and Max would have found some solution by now..
I actually logged out and in again when I've changed the phone, that was when I first had this problem (not being able to confitm my purchase), because Poweramp still was working on my old phone until I logged out, and now that phone has definitely died. And I'm afraid it would not work from any other Google account since the purchase in Google Play is vinculated to a certain Google account (and available on different devices as long as you're logged in from this certain Google account). So still looking forward to Max's reply... ๐๐ปโโ๏ธ