Sadly its a cross-browser issue, while other flash games still work for me on Kongregate, only Naruto Online fails to boot

Ok, a bit far fetched but Win 10 + Nox emulator + Puffin browser actually works. :DD

You need to set Puffin to force desktop version of sites (website preference setting),
so emulating desktop on mobile while emulating mobile on desktop solves the problem - but its far from ideal.