Weird width issue

Hey guys!
So I’m trying to get my responsive website to always take up 100% of the viewport width.
But for the life of me, I can’t figure out why some parts end up being a tiny bit wider and a scrollbar shows up.
Here’s a read-only link: Click me
And a little screenshot of the issue:
Thanks in advance!

Anyone? This is so simple (or at least it should). So simple I feel bad for even asking… Surely there’s someone out there who has an answer for me.