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

I have spent about 2 days on this problem now after WordPress updated to 5.7 on April 20, 2021. It was working perfectly before. It is still not working after everything recommended here. I also purchased an SSL certificate around the same time, so for a while I was thinking that it’s some sort of a conflict with that. I HAVE been able to narrow down that it’s the TYPE of link it doesn’t like and it is the Custom HTML block that is causing the issue in block editor. When I tried putting a simple piece of html in there it worked fine, but it is blocking my affiliate links from Amazon. One is a div, and one is is iframe. However, I can use a standard http link and link it to Amazon without problems.

This problem does not appear in classic editor at all which makes me think the issue is with the block editor and not with the link.

So for now, my workaround is using classic editor and adding the links directly to the code. I cannot go back to block editor after to edit because it will destroy the links in the document and fail to save and publish. What a frustrating experience and a huge waste of time, especially when everything was working fine prior to this update.

You should switch PHP7.4-FPM back to PHP7.2-FPM.
There’s nothing wrong with Permanent Links.
If you don’t use Apache 2.4, then NGINX server block should be checked for duplication and for my configuration. The correction is below:
server {
…
location ~ \.php$ {
include snippets/fastcgi-php.conf;
fastcgi_pass unix:/run/php/php7.2-fpm.sock;
#NOTE: You should have “cgi.fix_pathinfo = 0;” in php.ini
include fastcgi.conf;
fastcgi_intercept_errors on;
}

location ~ /\.ht {
deny all;
}
location = /favicon.ico {
log_not_found off;
access_log off;
}

location = /robots.txt {
allow all;
log_not_found off;
access_log off;
}

location / {
# This is cool because no php is touched for static content.
# include the “?$args” part so non-default permalinks doesn’t break when using query string
try_files $uri $uri/ /index.php?$args;
}

location ~* \.(js|css|png|jpg|jpeg|gif|ico)$ {
expires max;
log_not_found off;
}
}
This should be appropriate configuration to run your worpdress safe and sound. Good Day.

i have already used the clastic editor on my Blog but the error kept up coming

Changing permalinks also work for me and I get back my client website up and running

On my case, suddenly i was unable to post blogs. I was trying to find the issues and go to the htaccess file..I found there many codes related with wp rocket plugin. After deactive wp rocket it not solve. then i take a backup of my site and remove the .htacces and create a new htaccess file . Banggg, it solved i this post (https://taslimul.com/restoro-review/ )become live. :)ta