HELLO FELLOW TRAVELERS OF THE INTERNET TUBES.
I've started working with WordPress as part of my internship (I'm in week 3; adventure!) and it's ... not n00b friendly.
But, because so many people use it, there are a lot of responses when you google why something isn't working!
For THIS post: I KEPT GETTING THIS ERROR AND I (STILL) DON'T KNOW WHY.
That screenshot is via Wordpress 786 because I derp'd and didn't take my own before fixing it.
DOCUMENT YOUR ERRORS, PEOPLE.
The solution also came via Wordpress 786 --- Look on the left side of your screen and scan down until you see "Settings." On the menu that comes up on hover, click on "Permalinks".
Once that opens: don't change anything
Just go to the bottom and click "save."
Then, when you return to your post - poof the JSON error is gone.
Why does that work? I HAVE NO IDEA. I'm at the "I just need things to work" level of using WordPress. Someone who actually knows: feel free to hop in the comments. I'll be here to give you a thumbs up.