Good day, dennis-boyd1.
Thanks for joining the Community. I'm here to help resolve the JSON error.
The error message that you've encountered typically occurs if the JSON file is unable to transmit data across a network. Let's rule out this issue by using an incognito browser.
If you're able to edit the customer's detail, I suggest clearing the cache of your regular browser. You can check out this article for the detailed steps: Clear my browser cache and temporary Internet files.
That should do it. Let me know how it turns out. I'm always here to help.