Ionic 5 was announced a few hours ago (12th Feb 2020) and I upgraded my one of the small production app to Ionic 5 along with Angular 9:
Try ng serve --aot, it helped me fix the problem, that's if you want to run with aot which is recommended since it will be similar to the production build and it will help you catch errors sooner.
ng serve --aot
Hope this helps.
Angular Link: https://angular.io/guide/aot-compiler