Cannot run arduino as web client
This could be an SD card SPI bus issue. 

Note that because the W5100 and SD card share the SPI bus, only one can be active at a time. If you are
using both peripherals in your program, this should be taken care of by the corresponding libraries. If
you're not using one of the peripherals in your program, however, you'll need to explicitly deselect it. To do
this with the SD card, set pin 4 as an output and write a high to it. For the W5100, set digital pin 10 as a
high output.


  Check the Arduino shield datasheet.


Changing pin mode:

pinMode(4, OUTPUT);
digitalWrite(4, HIGH);