Thank you very much Francis!!! Your indications solved this problem:
with vue.config.js :

// vue.config.js
module.exports = {
// options...
publicPath: '',
devServer: {
host: '0.0.0.0',
port: 8080,
public: 'ggc.world'
},
}

now it works fine: https://drive.google.com/open?id=1PUctgdYLoVmJRvYyG040BFNGOev2yhRX

The previous problem looked similar but I guess it was somewhat different, because it disappeared once moving from Server Edition to Desktop edition of Ubuntu 18.04.4 .

Thank you very much again for your kind help.

Marco