Error events are generic errors that hide the underlying cause:

[object ErrorEvent] thrown

To expose the root cause of a generic error, implement a new test:debug script:

1. Implement test:debug, as shown, in package.json:

package.json
...
"scripts": {
  ...
  "test:debug": "ng test --sourcemaps=false",
  ...
}

2. Execute npm run test:debug
3. Now the Karma runner will likely reveal the underlying issue
4. If necessary, follow the stack trace to find the child component that may be causing the issue