The response is not a valid JSON response

Hi, when I update posts, I get this error:

Updating failed. Error message: The response is not a valid JSON response.

I’ve deactivated all plugins except tagDiv ones. This is my php configuration:
PHP Version: 7.1.33
memory_limit 256
max_execution_time 300
max_input_time 300
post_max_size 256M
upload_max_filesize 128M
opcache.enable on
disable_functions opcache_get_status

I forgot to say that this error occurs only updating posts, but not updating pages. It I check the DevTools Console, the next error message is displayed:

api-fetch.min.js?ver=3.6.4:1 POST http://mondraberri2.adoxkomunikazioa.com/index.php/wp-json/wp/v2/posts/78?_locale=user net::ERR_EMPTY_RESPONSE

Hi Calin, I updated my site as you say and now it works correctly and the JSON error has disappeared

Thank you very much for your help
Ramon