Finally I got dfu-programmer working:
$ sudo dfu-programmer at32uc3b0256 get
Bootloader Version: 0x02 (2)
$ sudo dfu-programmer at32uc3b0256 erase
$ sudo dfu-programmer at32uc3b0256 getfuse
Locked regions: 0xffff (65535)
Thanks Raman Gopalan for help!