Can’t Check Out From Cart Page (Empties Cart)

I have a strange situation.

Checking out works when going directly to check out (either from the “Checkout” menu button or selecting “Checkout” while hovering over the cart symbol in the upper right corner.

If you view cart first and then click “Proceed to Checkout” or click the Checkout button in either location (top menu or hovering over the cart symbol), then the cart empties and you’re presented with the message that you have no items in the cart.

When I switched the theme to Storefront (WooCommerce default), everything worked.

I currently have the site down, trying to figure out what is going on. The store has been up for 2 years and we don’t recall ever having this issue. We haven’t added any new plugins and everything is currently updated.

Any ideas of what might be happening? Again, you can check out as long as you don’t go to the cart before attempting to check out.

Thanks for any help. ??