Heat vegetable oil in a large pot and stir in the onion and celery, continuing to stir for about 5 minutes, before adding the potatoes. Cover and cook over low heat for about 10 minutes, and stir in the herbs. Mix the flour with milk to make a thin paste, then add the milk and broth to the soup pot and bring to a boil. Simmer until the vegetables are tender but not mushy. Stir in the chicken and optional corn, and sprinkle lightly with salt and pepper to taste. Cook gently 5 minutes; serve hot.