C++ std::string Zuweisung erzeugt Fehler!

//includes ...
const int SIZE_X = 50;
const int SIZE_Y = 20;
const char leer = 255; // ' '
std::vector<std::string> screen{SIZE_Y};
int screen_x = 0;

std::array<std::string, 20> level00 {
	"                                                                                                                                             ",
	"ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo",
	"o                   |                                                                                                                       o",
	"o                   |                                                                                                                       o",
	"o                   \\                                                                                                                       o",
	"o                    \\                                                                                                                      o",
	"o                     \\                                                                                                                     o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"o                                                                                                                                           o",
	"ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo"
};

int main() {
// ...
  initScreen();
// ...
}

void initScreen() {
	for ( int index = 0 ; index < SIZE_Y ; ++index ) {
		for ( int i = 0 ; i < SIZE_X ; ++i ) {
			screen[index] += leer;
		}
	}
}

void drawScreen() {
	for ( int i = 0 ; i < SIZE_Y ; ++i ) {
		for ( int n = 0 ; n < SIZE_X ; ++n ) {
			screen[i][n] = level00[i][(n+screen_x)];
		}
	}
    std::string titel = "TITLE HERE";
    std::string nummer = 123;
    std::string header = "  LEVEL: " + nummer + " - " + titel;
    screen.at(0) = header;
}

std::string titel = "TITLE HERE";
std::string nummer = std::to_string(1);
std::string header = "  LEVEL: " + nummer + " - " + titel;
screen.at(0) = header;

screen.at(0) = header;

screen.at(0) = "HEADER HERE";

58. std::string nummer = 123;

std::string nummer = std::to_string(12345);

        std::string titel = "TITLE HERE";
	std::string nummer = std::to_string(1);
	screen[0] = " ";
	screen[0] += "Level: ";
	screen[0] += nummer;
	screen[0] += " - ";
	screen[0] += titel;