How to Fix “Updating Failed” in Gutenberg
It should be fully clear what the issue is if you look on the screenshot below:

On this screenshot you can also see that I am using custom background and custom fonts. If you’re interested in how to do that, check this tutorial.
Coming back to our issue, it happened just after I moved my website to localhost.
Here is a couple error messages you can find in the browser console:
- “Uncaught (in promise): The response is not a valid JSON response.”
- “Uncaught (in promise): Not found”
- “Failed to load resource: the server responded with a status of 404 (Not Found)”

I think many of you already know how to fix this issue 😁 Have some ideas?
If not, my next question is – do you use permalinks (pretty URLs) on your website? 🤔
Yes, yes, all you have to do is to go to Settings > Permalinks from WordPress admin menu and hit the button “Save changes” without changing anything. That’s all!
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Misha Rudrastyh
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Really thank you.
thanks