Ok, totally began from scratch by installing the QuickStart again - I ended up with the exact same problem that sticky nav on default homepage (and timeline, contact us where landing layout is used) didn't work.
The console looks like this (what I'm wondering is the 404 on a css file here)?

I then realized that uBlock Origin Chrome Addon was active on my dev site. So I whitelisted the dev site in uBlock and voila the sticky nav is working now!
This brings in some questions:
1) What does this addthis js code is doing exactly? Is it "only" a tracking widget used by joomlart to see how many people are using purity?
2) You see uBlock and other 'tracking blocking addons' block this addthis* js code which causes sticky to not work at the beginning - which is a bad thing imho. (now that I whitlisted the dev site one time so sticky nav worked the sticky nav still works when I enable uBlock for this site again)