It looks like you're not setting your android-versionCode in your app. If you attempt to overwrite an application without incrementing the android-versionCode, it will fail. Either add an android-versionCode, or uninstall the existing version prior to installing.
Thank you Ryan, indeed setting the Android versionCode does the trick.
I'm somehow surprised as it's been more then 3 years, that I've been building this app on pgb and I've never had to set the versionCode.
Still it's great to see it work again so I will increment the versionCode from now on ;-)