This looks a bit like this case here: PGAdmin: Not connected to the server or the connection to the server has been closed
Output of columns of type json seems not to work.
SELECT normal_column FROM table
Runs fine
SELECT * FROM table (or SELECT json_column FROM table)
Outputs: "Not connected to the server or the connection to the server has been closed."
But the notification window flashes the correct "Successfully run. N rows affected".
Did not have this problem with pgadmin 4.2 or 4.1.