Checkout Page Issue – Coupon Code

I just found something weird with my site.

My site seems to function great with adding parts to the cart and when customer needs to increase Quantity from 1 to 2 by clicking the green + there is not a problem.

But if a customer types in the COUPON Code and then tries to Increase or Decrease the Quantity the system EVERY TIME in EVERY Browser brings up a Pop up that is taking customer away from Checkout Page.

See attached Pop Up.

I do not want to post a LINK but could email the Link to anyone you may be able to help with this issue.