The issue is very likely related to the reverse proxy set in the live website. We could confirm that by analyzing a staging version and a local copy, where the issue is not happening. The local copy is an exact fresh copy of the live website.
Background of the issue:
I am trying to translate a page/article into another language (French, German, Spanish, etc.).
Symptoms:
An error message appears: 'Updating failed. The response is not a valid JSON response.' I can't save or edit.
Questions:
Why am I receiving a 'Updating failed. The response is not a valid JSON response.' error?
How can I resolve this issue to save or edit translations?
Can you please provide as much information as possible, on how the proxy was set up?
"I would recommend that the client have dialogue with their host check the proxy settings as well as to check the access logs for their proxy server to see the routing of the connections."