Drupal – Ajax error 403 entity too large, how to fix

The issue, at least in my case was in the NGINX configuration
Usually you have to modify the file:
/etc/nginx/nginx.conf
You have to add/modify this value:
client_max_body_size 100M

Usually you’ll find this parameter in this context:

http {
[more stuff here]
client_max_body_size 100M;
[more stuff here]
}

But it might be somewhere else if you use for example Yunohost or probably even Sandstorm.
So in that case file might be located here:
/etc/nginx/conf.d/yoursite.tld.d/yourapp.conf

And in this case it should go inside the location thingy, like this:

location / {
  client_max_body_size 100M;
  [stuff...]
  [more stuff]
  [actually a lot of stuff]
}

As this article is more like a reminder for me I think this is enough information, if you, eventual visitor, will read this and want to correct me, add info, etc. Your comments are welcome, I’m sure it’ll even help my indexing, so thanks.