pgadmin4 can't start

I'm working on debian buster

I installed pgadmin4-desktop following instruction:

https://www.pgadmin.org/download/pgadmin-4-apt/

from terminal

cd /usr/pgadmin4/bin
./pgadmin4

Python path: "/usr/pgadmin4/venv/lib/python3.7:/usr/lib/python3.7/lib-dynload:/usr/pgadmin4/venv/lib/python3.7/site-packages"
Python Home: "/usr/pgadmin4/venv"
Webapp path: "/usr/pgadmin4/web/pgAdmin4.py"
Failed to connect to the server: "Connection closed" ( request URL: "http://127.0.0.1:5432/misc/ping?key=f600e8c7-9b4e-45f7-88c7-88e6fce18345" ).
Failed to connect to the server: "Connection closed" ( request URL: "http://127.0.0.1:5432/misc/ping?key=f600e8c7-9b4e-45f7-88c7-88e6fce18345" ).
Failed to connect to the server: "Connection closed" ( request URL: "http://127.0.0.1:5432/misc/ping?key=f600e8c7-9b4e-45f7-88c7-88e6fce18345" ).
Failed to connect to the server: "Connection closed" ( request URL: "http://127.0.0.1:5432/misc/ping?key=f600e8c7-9b4e-45f7-88c7-88e6fce18345" ).
Failed to connect to the server: "Connection closed" ( request URL: "http://127.0.0.1:5432/misc/ping?key=f600e8c7-9b4e-45f7-88c7-88e6fce18345" ).
Failed to connect to the server: "Connection closed" ( request URL: "http://127.0.0.1:5432/misc/ping?key=f600e8c7-9b4e-45f7-88c7-88e6fce18345" ).
Failed to connect to the server: "Connection closed" ( request URL: "http://127.0.0.1:5432/misc/ping?key=f600e8c7-9b4e-45f7-88c7-88e6fce18345" ).
Failed to connect to the server: "Connection closed" ( request URL: "http://127.0.0.1:5432/misc/ping?key=f600e8c7-9b4e-45f7-88c7-88e6fce18345" ).
Failed to connect to the server: "Connection closed" ( request URL: "http://127.0.0.1:5432/misc/ping?key=f600e8c7-9b4e-45f7-88c7-88e6fce18345" ).
* Serving Flask app "pgadmin" (lazy loading)
* Environment: production
WARNING: Do not use the development server in a production environment.
Use a production WSGI server instead.
* Debug mode: off

and then hundreds of rows like:

Failed to connect to the server: "Connection closed" ( request URL: "http://127.0.0.1:5432/misc/ping?key=f600e8c7-9b4e-45f7-88c7-88e6fce18345" ).

What's wrong in your opinion?

Pgadmin3 works well