Well it turns out the problem lies somewhere in my hosts installer. I use installer because my upload speed with my ISP is really slow. So I use installer to install fast. But this was not a good choice.
So I installed manually and this mod is working now, without having to clear Theme and SASS cache.
But even when I do clear Theme and SASS cache, it does not crash my site anymore.
So the problem is definitely with my hosts installer.
I installed this (through upload installer, 188.8.131.52, default theme) and it half worked. Products that are out of stock will no longer add to cart, but there is no message stating that it's out of stock. Any idea why? I'd appreciate any guidance you can give!