"The response is not a valid JSON response" error

Hello,
I'm getting "The response is not a valid JSON response." error whenever I try uploading an image directly from the post editing page while using Gutenberg; and the image won't load. It is the same with a single image block or a gallery block in Gutenberg. When I first upload the image to media gallery, no problem happens and the image is attached to the post as it should be.
I couldn't find anything on your support forum. When I searched the internet, I came across some posts implying that it can be an issue between Gutenberg and site theme. I don't know if it is true or not, but I wanted to ask you.
Thank you.
Wordpress version 4.9.8
Gutenberg version 3.6.2
GP Premium Version 1.7.2
Site is running on a Windows server with Php 7.1.20 (So, Windows Compatibility Fix Version 1.0.2 is installed on my Wordpress.)