Hello.
When I add thing to card and click to “checkout” (immediately) shows a notification:
I delete cookies in my Firefox and it doesn’t work. My friend have the same problem.
From what I understand, your 20min simply run out. Try to complete your order within those 20min, not longer.
I click “checkout” immediately! Even one minute doesn’t run out! I’m not stupid. 
EDIT
Ok, I get it! In my address I have polish letter and this is it. When I delete that letter everything is work. 
Glad that it works now!