[Chiuso] JSON error after saving the page

Background of the issue:
I am trying to create a translation of a page in JA language using WPML. There is no possibility to save it, and I believe there is a problem with the Japanese signs. The translation in JA language was created but cannot be saved unlike all other languages. I need to save the page in any language when I edit some content inside or add a block. I was advised to make changes in the original language and translate using WPML editors. Link to a page where the issue can be seen: link nascosto | link nascosto

Symptoms:
When I try to save the translation in JA language, it throws a JSON error. Instead of normal behavior as in all other languages, I got: "Updating failed. The response is not a valid JSON response."

Questions:
Why can't I save the translation in JA language?
What causes the JSON error when saving the page?