How to Fix “The response is not a valid JSON response” error?

When I adjust the widgets of my website built in WordPress, I made some changes then to save it. However, it encounted the “The response is not a valid JSON response” error.

So, how to fix “The response is not a valid JSON response” error?

Solution for “The response is not a valid JSON response”

There are many reasons due the error. Although I didn’t know why it with the “The response is not a valid JSON response” error, I find a way to fix the error.

In my case, I just installed a class widget plugin.

https://wordpress.org/plugins/classic-widgets/

Then active it, the wordpress widget editor was changed to the class widget style.

By this way, I can changed the widgets now.