On the attached cart page, if you enter a valid coupon (in this case COUPON1) I get this error:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, email@example.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
If you use a coupon code that although is in the db, is not active (eg COUPON3) the page reloads fine
Have attached the cart page and link in PM