I found the solution :)
I wrote a test program and found a working baud-rate at 600.

My test program:
long baudrates[] = {600,1200,2400,4800,9600,14400,19200,28800,38400,56000,57600,115200,128000,256000};
unsigned char baudcounter = 0;
void setup() {
Serial.begin(baudrates[baudcounter]);
}
void loop() {
Serial.println();
Serial.println(baudrates[baudcounter]);
Serial.println(" !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~");
Serial.println();
baudcounter++;
baudcounter %= sizeof(baudrates)/sizeof(long);
delay(1000);
Serial.begin(baudrates[baudcounter]);
}