Message "Discount code already used"

-- HikaShop version -- : 2.6.1
-- Joomla version -- : 3.4.8
-- PHP version -- : 5.4.44
-- Browser(s) name and version -- : Firefox 44.0.2

Some time ago I imported some discount codes. Since then, when I want to create a new code I get the message like "Diskount code already used" but the code is created sucessfully. Also on editing I get this message except with the older codes before my imported codes.
What my I have done wrong?
Best regards

Hello,
Did you checked that your didn't set a value already used by existing discounts through your new discount's "Code" option ?
Can you tell me how did you exactly imported your "older codes" ?

I tried it with a new not existing code but got this message.
The first codes I created with hikashop. Some time ago I imported around 400 codes direct in the SQL database. Since then I have this problem.

I checked the table xxxx_hikashop_discount and everything seems to be ok. I created a new code just hitting some buttons "kjhermnb83ghdkbd7" and searching the table for this code if there is one. If not, I created a new discount in hikashop with this new code and got the message that this code is already used. It is not a major error. The code and every entry I made is created sucessfully. I can live with this message but it is a bit confusing for other who want to create discount codes.