If any of you want to see a real fix, make sure to confirm this bug:
https://bugs.kde.org/show_bug.cgi?id=375016
And comment that the last known good commit is what is used here now:
https://github.com/KDE/k3b/tree/8102d390e763abc1e4db2954798930e5f04d6370
K3b unable to add files
Just done! Thanks!!
Cheers
Paolo