In particular with this movie: tt0019154 from external_source=imdb_id

That is:

https://api.themoviedb.org/3/find/tt0019154?api_key=<my-api-key>&external_source=imdb_id

yields:

{
  "status_code": 11,
  "status_message": "Internal error: Something went wrong, contact TMDb."
}

Other than that, most other movies work for me!