The editor did not receive the JSON response it expected; the message does not identify the root cause. Preserve your unsaved text before refreshing. Otherwise, troubleshooting may lose the work you are trying to save.
Inspect the failed request
Open the browser’s Network panel and repeat the save once. Check the status, response type and body. A login page, server error or security-layer response can arrive where the editor expected JSON. Reading that response is more useful than installing a plugin based only on the error wording. 3
Confirm that the WordPress session remains valid. Editor REST requests depend on user permissions and authentication; an open admin tab does not prove that the next request will succeed. 1
Check the server-side evidence
Review REST API notices in Tools → Site Health, and ask the host to correlate the failure time with server logs when necessary. 2
Do not disable every security control on the public site. If a particular rule blocks a legitimate request, fix that rule narrowly and verify the result.
For a regression following an update, reproduce it on staging with one change at a time. Finally, save again and inspect the public page. The disappearance of an error is not, by itself, proof that the intended revision was published.
Related reading
WordPress maintenance and performance
