Won't apply coupon code

-- url of the page with the problem -- : www.panosfx.com/commercial-free-photoshop-actions/filmstrips
-- HikaShop version -- : 2.6.4
-- Joomla version -- : 3.6.2
-- PHP version -- : 5.5.38
-- Browser(s) name and version -- : Chrome latest, Firefox latest

It's been more than 1 month that I am encountering this strange issue. It happens sporadically, yet quite often.

I am adding a product that has a coupon code, to the cart, and I proceed to checkout.
I enter the coupon code and click on "Click to add". Nothing happens. The page is refreshed but the coupon is not applied.
If I try to enter the coupon again, the page is refreshed but this time the product is removed from the shopping cart!
If I try for the third time, it works. Strange.

You can check it on this page:
www.panosfx.com/commercial-free-photoshop-actions/filmstrips
Try to enter the valid coupon code: orange

Thanks
Panos

I just tried it again and encountered the same issue
I am attaching 2 screenshots:

Hello Nicolas.
What's strange is that it happens sporadically (not always, but rather often):
I tried to reproduce it now and in all instances it worked as expected.
But yesterday the coupon would often not apply, no matter:

• the browser that I used (Chrome, Firefox, Opera, on my PC), with cache cleared
• if I had logged in prior to proceeding to checkout, or not.