The error is as follows:
When creating a new address (for example for a sales order) the app tries to find the ZIP-code from the city.
It should be the other way around.
If the ZIP code and city already exists in the database, it works properly. Meaning that when you write the zip code, it will writ the city for you.
However.
If the ZIP code and city don't exist in the database, it will do nothing when you write the zip code. Then, when you fill in the city it will open a window where you can pick one of the existing zip-codes associated with that city and overwrite the one you entered.
This is a logical error, trying to find a unique ID from non-unique data.
Like trying to find the licence plate by describing the model of the car.