lumia 520 boot repair problem

i have one nokia 520 rm-914
it get detected by pc like nokia..but atf say non 8.1 windows device detected.
i use latest version software.
when i try to repair boot..it detects target…but core halt error
here is log:
Halting Core with 100ms Delay…Cannot Halt Core!
Resetting and Exploring JTAG chain…2 Device(s)
Device #1 ID 0x4BA00477 IR4
Device #2 ID 0x207D00E1 IR11
Restore Boot…
Waiting for Device Connection 15 second(s)…OK
Resetting and Exploring JTAG chain…OK
Halting Core with 100ms Delay…Retry
Resetting and Exploring JTAG chain…OK
Halting Core with 100ms Delay…Retry
Resetting and Exploring JTAG chain…OK
Halting Core with 100ms Delay…Retry
Resetting and Exploring JTAG chain…OK
Halting Core with 100ms Delay…Cannot Halt Core!
any sugestions apreciated