I occurred to the problem that I had developed a custom activity on aws EC2(OS: CENTOS 7, Enviroment: Node.js + pm2 + nginx) and had connected back to SFMC installed package.
However, when I create a new Journey in the Journey Builder, my custom activity could't display the icon and I got a "Failed to load config.json for url..." message.
But I'm sure that I can connect to config.json through my local sever.
Any further troubleshooting assistance or similar is greatly appreciated. Thanks so much!