Add
        client_max_body_size 30000M;
        client_body_buffer_size 200000k;
this 2 options in etc/nginx/sites-available/default or etc/nginx/nginx.conf whichever is applicable

Hi John,

answer is a bit late, but for the sake of closure: this problem is related to your NGINX config. As you know, NGINX config is a little bit tricky, but basically you can enter parameters on http, server and location levels. So somewhere in your NGINX config (e.g. in /etc/nginx/nginx.conf for http level or in /etc/nginx/sites-enabled/<HOSTNAME>.conf for server level, look for this line:

    add_header X-Frame-Options DENY;

and replace it with this line:

    add_header X-Frame-Options SAMEORIGIN;

and restart nginx. Remember to check the nginx documentation, bcause if you change this setting on http level, it will affect other sites on the server as well, and if you add the add_header command on server level, nginx will ignore all add_header directives that you entered on http level.

Solution drawn from: https://github.com/odoo/odoo/issues/18137#issuecomment-459706730