No…
No special characters are used. I’ve tried several ways:
5 digits with dash. (In Poland postal codes are usually given in the -* form);
5 digits without any dash character;
“PL” in the front and 5 digits, both with and without dash…
This must be an error in the app.
And “Kosowa”? I have no idea where the app took it from. There’s even no street in the neighbourhood with that name.
After I click “Next” it displays “Please fill all fields in the form”.