>>302011772
I had this with the Oculus Rift S and when I got the Valve Index I still get it. I believe it's just the shape of our heads. Maybe we have big heads. I tried a lot of things and usually carefully adjusting the top strap can get rid of the headache but not entirely. I'd guess it has something to do with sinuses too. Some have tight ones, some don't and depending where pressure is applied on your head it makes it worse and induces headache.

If you ordered the full index set, then try using the soft padding meant for little heads but set the straps and everything loose and then tighten.