As you mentioned in your message, there seems to be a conflict between virtuemart bonus and awocoupon. And on disabling either awocoupon or virtuemart bonus, the error goes away.
I looked a little more into vm bonus, and it looks like it is running its own "setcoupon" override. I assume the clashing of awocoupon trying to set the coupon and vm bonus trying to set the coupon is causing problems. So I have enabled vm bonus on your site, and in the file:
/public_html/plugins/system/bonus/bonus.php, around line 73, I commented out the original and added my line of code:
//$tasksToOverride = array('add','addJS','setcoupon','delete','update','confirm','checkout','updatecart');
$tasksToOverride = array('add','addJS','delete','update','confirm','checkout','updatecart'); # seyi_code
Basically, I remove the "setcoupon" task which means vm bonus will not process it any more and only AwoCoupon will. I am not sure what adverse effects this could have on vm bonus, so just check to make sure that is still working correctly.