Many thanks for both answers. I've tested every pin and as far as I can understand it seems fine, but still no answer from the Ethernet.
What do you mean by "everything else works." What other tests have done to make sure your code is working fine? What code are you using?
I ment that any simple code was working fine (blinking leds, controlling servos, reading temperature sensors, accelerometer, etc), until weird symbol readings started to appear at my monitor serial and suddenly I couldn't receive anything at all (at serial). I'm using a USB to serial adapter.
After not being able to receive anything, I've unplugged only the 5v pin from the serial adapter and used and external power, then serial readings where working again (seldomly with weird symbols as well).
Is my Arduino dying slowly? xD
WS5100 is getting really hot, but I think it works at relatively high temperature (around 80ºC?)
Many thanks again!! (and sorry for my english)
Edit: I forgot to say that I tryed differents ethernet cables and also nothing.