GRunner80 on "[Plugin: Piraeus Bank Greece Payment Gateway for WooCommerce] Illegal string offset"

Hello,

Thank you for this great plugin! It works great so far...
I am in the process of doing the test cases. Each time I complete a test case and be redirected back to my site, I get this message at the top of the page

Waing: Illegal string offset 'message' in /public_html/wp-content/plugins/woo-payment-gateway-piraeus-bank-greece/woocommerce-piraeusbank-payment-gateway.php on line 556

Waing: Illegal string offset 'message_type' in /public_html/wp-content/plugins/woo-payment-gateway-piraeus-bank-greece/woocommerce-piraeusbank-payment-gateway.php on line 557

I am using Woocommerce Version 2.5.0

https://wordpress.org/plugins/woo-payment-gateway-piraeus-bank-greece/

Do you have debuging enabled?

No,
I have checked wp-config.php and the WP_DEBUG is set to false...