Received Updateing failed. not a valid JSON response. linked with _locale=user

I’ve been struggling for few days to resolve the issue with “Updating failed. Non-valid JSON response message at the top of my page.

The issue comes with the image block and the icon box block from Gutenberg. When I remove the blocks the issue is resolved. I’ve tried everything I found on the net: deactivating all the plugins, saving permalinks structure, modifying htaccess, whitelisting IPs and json paths…nothing seems to work. And what is worse, I do think that the page is updating, but the message is there.

can anybody help?

the issue is linked with the following path : /wp-json/wp/v2/pages/115%3F_locale=user and it gets 500 error code in the debug.

thanks