Redirection after successful payment – blank screen

First of all I would like to thank you for sharing this plugin and congrats of the nice job that you have done.

After the bank send me the test account, I tried to make the test and for the cancel and fail the plug in was working perfectly. But for the success case, I face the following problem.

After a successful payment the bank confirms the payment by delivering success email but the customer of mine redirects to a blank screen at the url that I provided http(s)://www.yourdomain.gr/wc-api/WC_Piraeusbank_Gateway?peiraeus=success and the status of the order remain at “pending payment”

I have already conduct to the bank to ensure that the link is correct and they replied that the url is the correct one and the test have been successfully passed.

Do you have any idea which can be the problem or what I have done wrong.

Many thanks in advance.