This is perfect, you are very helpful. It has been solved.
How it was solved?
As you suggested, I disabled caching in the “Reduce server response time” section of Advanced settings of PageSpeed Ninja. The error goes away.
I re-enabled the cache, the error came back again. Then, deactivated “WP Super Cache”, it disappeared, and re-enabled “WP Super Cache” the error didn’t show again.
Thank you.
