fail over button failed to fail over

Created an attachment (id=572)
diag file

Create a cluster of 3 nodes of ns server 1.6.0beta3.1-15 on centos 5.4 64bit.
http://screencast.com/t/MDRiODBmY
Shutdown 2 nodes by uninstall server (node A and B with IP ending 59 and 60)
If I click fail over of node B, the page will reload with error.
http://screencast.com/t/ZDhhYjJlN2
Click fail over of node B again, still show error
http://screencast.com/t/YWY2NWM4Mjc
Error in log page
http://screencast.com/t/OTI0ZTAzY

If I click fail over node A first, then on node B (follow the order from top to bottom), the process of fail over works and I can click rebalace to finish the work.
http://screencast.com/t/YjEyYjUw