Steps to reproduce:

View any dashboard in the frontend

Result:

You will receive a 404 not found for the path assets/styles/vendors/Leaflet/Leaflet/leaflet.css

Expected:
We expect any files in the vendors folder to be accessible by the frontend.

The following section in /etc/zabbix/nginx.conf is preventing access to the leaflet.css file:

        location ~ /(api\/|conf[^\.]|include|locale|vendor) {
                deny            all;
                return          404;
        }

If we remove "|vendor" from the location line and restart NGINX we are able to load the leaflet.css file.

I am assuming that the vendor entry above is supposed to match /usr/share/zabbix/vendor but not /usr/share/zabbix/assets/styles/vendors so the entry will probably need to be updated.