Can't upload sketch

Hi all,

I'm making a little project with Arduino Yun, after some round of modify/upload/test of my sketch upload process suddenly gave me this:

avrdude: verification error, first mismatch at byte 0x0000
         0xff != 0x0c
avrdude: verification error; content mismatch

Thinking about some problem with my code I tried to upload an empty sketch but result was the same.

Can somebody give me an hint? I googled around without luck.

Many thanks
Fabrizio

How have you connected your Yun to your PC?
Via USB or Wifi?
Have you tried a different port?
Have you rebooted your PC?

My test was in wifi only, as I wrote all works right for days and suddenly I get the described error.
Tonight I'll try by usb.

Many thanks
Fabrizio

Have you upgraded openwrt-yun? (look at my signature)
If so, uploading via wifi should fix the issue

No I haven't upgraded it, it's safe to upgrade it via WiFi?

Yes, it is. The link in Federico's signature is the official tutorial to upgrade the board.