Answer
#1 – No Cache plugin found. My Woocommerce cart is working fine. Only This one get HTTP500 issue.
#2 – Called up my Hosting company and they said, it’s fine.
#3 – Same result on existing site. But, I created a new temp site just to test this and it worked fine. No issue after second click on Add to cart. What could be problem with my first site.
Any advice?