Shopping cart not emptying after customer completes check out

Plug-in is working excellent, however there is an issue I have ran into where the cart is not emptying after the customer completes check out.

I disabled the plugin and the problem went away.

There is also a fatal error log:

NOTE: I am trying to purchase your Wholesale Plugin Premium (not the bundle, just this 1 plugin) on your website, but at check out screen it keeps removing the 50%off discount, I am not sure why?