For me, when I disable Woocommerce Poor Guys Swiss Knife and SUMO Reward Points, things began to work. I can live without Swiss Knife, but Reward Points (from CodeCanyon) is a necessity, so I take this solution as a temporary work-around.
If any of you find the solution that works, please post!
EDIT:
Also, I restored my backup BEFORE WC database update. Same issue before and after.
If you are logged in, it appears the add-to-cart works. But if you are not logged, now it is throwing error 500.
I just want to pin point what is going on. My business is now came to a halt.