Ah. I found the problem: I tricked myself. I didn't delete the free version from the Applications folder, I just renamed it (that seemed like a good idea at the time …).
Deleting the app before installing the App Store version solved the issue, now everything works as expected.

Thanks for your ...

Merci beaucoup pour votre réponse! (that's about all my French [:D])
Yes, I tried that, a couple of times, sometimes I deleted the not-quite-installed program file in the applications folder beforehand, sometimes I left it there. Each time with the same result.

https://datavoir.de/index.php/s ...

Hello everyone!
I have been using the free version for quite some time now, and I am very happy with it.
So, wanting to support the developers (and get the extra models), I decided to purchase the App Store version.
That went well apart up to the point where Launchpad says "Installing / 245.6 MB of ...