Angular Upgrade From 5 to 6: asset path must start with the project source root

Ok,
Clarifying Abann’s solution. You must use the long form for assets outside the sourceRoot. I do think the documentation could be much more clear on that. It states

You can use this extended configuration to copy assets from outside your project. For instance, you can copy assets from a node package:

But really, it should say you must use

"assets": [
          "src/assets",
          "src/favicon.ico",
           {
            "glob": "*",
            "input": "node_modules/font-awesome/fonts",
            "output": "assets/fonts/"
          }
        ],

Where assets/fonts/ is the path you probably configured in your SASS/SCSS file’s $fa-font-path variable override.