Thank you so much for your feedback!
As I said, I am new to this, it seems I will have to get an official keystore from Google so I can officially sign the apk. I do have Android Studio, but never got that far. Setting the default keystore, fixed it! (with security warning of course).
As for the v2, the APK download maintained the exact same filename, but then again, I only changed the keystore. I did however get a “failed build” mail now and after changing the keystore, I received the succesful build mail, as you mentioned earlier. Hope this feedback helps you and others. As you have undoubtedly seen, not being experienced does raise some questions.
PS: great work on your part, creating a platform for users to be able to create stuff. Beautifull !!!
Kind regards,
Michel