@eirinigiak hello. i would like to inform you that no changes have been made on the plugin so the problem is on your side.
We suggest that you 1st confirm with the bank that they have the correct links
WITH PERMALINKS SET
- Website url : http(s)://www.yourdomain.gr/
- Referrer url : http(s)://www.yourdomain.gr/checkout/
- Success page : http(s)://www.yourdomain.gr/wc-api/WC_Piraeusbank_Gateway?peiraeus=success
- Failure page : http(s)://www.yourdomain.gr/wc-api/WC_Piraeusbank_Gateway?peiraeus=fail
- Cancel page : http(s)://www.yourdomain.gr/wc-api/WC_Piraeusbank_Gateway?peiraeus=cancel
WITHOUT PERMALINKS (MODE=SIMPLE)
- Website url : http(s)://www.yourdomain.gr/
- Referrer url : http(s)://www.yourdomain.gr/checkout/
- Success page : http(s)://www.yourdomain.gr/?wc-api=WC_Piraeusbank_Gateway&peiraeus=success
- Failure page : http(s)://www.yourdomain.gr/?wc-api=WC_Piraeusbank_Gateway&peiraeus=fail
- Cancel page : http(s)://www.yourdomain.gr/?wc-api=WC_Piraeusbank_Gateway&peiraeus=cancel
Response method : GET / POST
IP: Your’s server IP Address
You will also need to pay attention to the following:
a) The order completion page on your Woo Commerce site should be in the format http://www.yourdomain.gr/checkout/.
You will need to edit the permalinks on your site and edit the URL path so that it is exactly as shown above.
b) Make sure that the selected Return Page is the Page: Thank you
Other than that , you will have to check the relevant logs if the problem still continues. Please check also verify that a new plugin that might have been installed or any other update might be causing this problem that your error logs may record .