Cortex-R5 target cannot be stopped

After DS-5 connected to target, "Debug Control" tab shows "Cortex-R5_0#1 running", but "Commands" tab shows

"ERROR(TAD9-NAL30):
! Unable to stop device Cortex-R5_0
! Cannot stop target."

in response to "stop" cmd. Any idea about it? what issue usually causes this? 

Thanks

James