Coinbase payment

This error appears when I try to set a coinbase payment :

An uncaught Exception was encountered
Type: Guzzle\Common\Exception\RuntimeException

Message: Unable to parse response body into JSON: 4

Filename: /var/www/html/invoiceplan/vendor/guzzle/guzzle/src/Guzzle/Http/Message/Response.php

Line Number: 861

The issue was previously evoked in another topic but this link seems to be down :

Maybe someone can help me ?

Thank you in advance and sorry for my bad english.