I was using avrstudio(6.0.1822)+mkII(7.19),the CPU is AT32UC3C2512C. They worked well.
After updated avstudio to 6.1.2730 and mkII to 7.24, the device id read by mkII is alwals 0x4200003F which matched with AT32UC3C0512C.
I tried two laptops, 3 USB hubs, more USB ports. It was same.
I had to down to avrstudio(6.0.1822)+mkII(7.19). It works again.
Please fix the bug.