SHT11 and Ethernetshield

Hello, I'm running following code on an arduino with ethernetshield:

#include <Ethernet.h>
#include <SHT1x.h>
#include <SPI.h>

byte mac[] = { 0x00, 0xAA, 0xBB, 0xCC, 0xDE, 0x01 }; // RESERVED MAC ADDRESS
IPAddress ip(192, 168, 2, 177);

EthernetClient client;

// Specify data and clock connections and instantiate SHT1x object
#define dataPin 51
#define clockPin 53
SHT1x sht1x(dataPin, clockPin);

void setup()
{
Serial.begin(19200); // Open serial connection to report values to host
Serial.println("Starting up");
}

void loop()
{
float temp_c;
float temp_f;
float humidity;

// Read values from the sensor
temp_c = sht1x.readTemperatureC();
temp_f = sht1x.readTemperatureF();
humidity = sht1x.readHumidity();

// Print the values to the serial port
Serial.print("Temperature: ");
Serial.print(temp_c, DEC);
Serial.print("C / ");
Serial.print(temp_f, DEC);
Serial.print("F. Humidity: ");
Serial.print(humidity);
Serial.println("%");
Ethernet.begin(mac, ip);
if (client.connect("XXXXX.de",80)) { // REPLACE WITH YOUR SERVER ADDRESS
client.println("POST /add.php HTTP/1.1");
client.println("Host: XXXXXX.de"); // SERVER ADDRESS HERE TOO
client.println("Content-Type: application/x-www-form-urlencoded");
client.print("Content-Length: ");
//client.println(data.length());
client.println();
client.print("temp=");
client.print(temp_c);
client.print("&");
client.print("humidity=");
client.print(humidity);
} else {
// kf you didn't get a connection to the server:
Serial.println("connection failed");}
if (client.available()) {
char c = client.read();
Serial.print(c);
}
if (client.connected()) {
client.stop(); // DISCONNECT FROM THE SERVER
}

delay(2000);
}

I don't get any correct values. When i delete the ethernet-client part in the code i get correct values. What could be the problem? The SHT11 is not connected to pins 10 and 11 which are reserved for the ethernet shield.

How do you power the system?
USB, batteries, wall wart?

Powered by USB.

This makes it look like you are using a Mega or Due.

#define dataPinĀ  51
#define clockPin 53

If you are using a Mega, those are the SPI bus pins. Don't use D50 to D53.

The ethernet shield doesn't use D11 to D13 for the SPI data lines. It uses the ICSP pins. That makes it compatible with the Uno, Leonardo, Mega, and Due.