Combine broth with flour in a saucepan. Cook over medium heat until thickened, stirring constantly. Add tomatoes with juice, chiles, onion, garlic, sugar and seasonings. Bring to a simmer, stirring frequently. Remove from heat.
Divide seafood and half of cheese evenly among tortillas; roll up and arrange seam-side down in a greased 15"x10" baking pan. Blend sour cream into tomato mixture. Spoon tomato mixture over enchiladas; sprinkle with remaining cheese.
Bake, uncovered, at 400 degrees for 15 minutes, or until cheese melts.