Angular 9: Value at position X in the NgModule.imports is not a reference

I upgraded an Angular App from v8 to v9. The project imports a custom UI library using Angular 8 and moment.js.

When I build it:

1. It generates a warning:

WARNING in Entry point '@myLib/catalogue' contains deep imports into
 '/Users/xyz/Projects/forms-frontend/node_modules/moment/locale/de'.
This is probably not a problem, but may cause the compilation of entry points to be out of order.

In the @myLib/catalogue.js file of the library (inside node_modules folder), the moment.js DE locale is imported as following:

import 'moment/locale/de';

2. Compilation errors are also triggered:

ERROR in Failed to compile entry-point @myLib/catalogue (es2015 as esm2015) due to compilation errors:
node_modules/@myLib/catalogue/fesm2015/catalogue.js:213:26 - error NG1010: Value at position 2 in the NgModule.imports of FormInputModule is not a reference: [object Object]

213                 imports: [
                             ~
214                     CommonModule,
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
... 
219                     TranslateModule
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
220                 ],
    ~~~~~~~~~~~~~~~~~

The text of the warning seems explaining exactly the compilation error, where the position (2 in this case) is out of range of the imports array.

I have seen different articles/github issues about deep-links, but no working solution.

In my case the issue was related to an imported library, that was not Angular v9 compatible (among other things it was not using deep links to Angular Material and moment.js).

I was lucky as the library is an intern one and I could fix these points and republish it. After that it built without any problems or changes on my project side.