This question has been answered by gissirob. See the first response.
[Update] Another user reported fixing this by deleting all files in the core/cache directory and viewing the site in an Incognito window. If that works for you, deleting the cache files and clearing the browser cache and cookies should fix things.
i had same error.
I found a solution editing one plugin code here : public_html/assets/components/articles/js/article/update.js
Changing the line
process: (MODx.config.connector_url) ? 'resource/update' : 'update'
After cleaning all caches it works fine.
NB: other plugin in your case might be in cause. Try to look for 'update' in the assets directory of your site, and check for all occurrence.
I hope it can help somebody.