request /pools/default/rebalanceProgress error timed out (cluster hangs)

I see 2 tests with the same symptoms:
1. http://qa.hq.northscale.net/job/ubuntu-64-2.0-upgrade/108/consoleFull
Rebalance exited with reason {important_nodes_went_down,
{ns_node_disco_events,
['ns_1@10.3.3.19','ns_1@10.3.3.24',
'ns_1@10.3.3.26','ns_1@10.3.3.27'],
['ns_1@10.3.3.19','ns_1@10.3.3.24',
'ns_1@10.3.3.27']}}
ns_orchestrator002 ns_1@10.3.3.24 00:14:51 - Thu Apr 18, 2013

see MB-8127, perhaps it is the same reason

2.http://qa.hq.northscale.net/job/ubuntu-64-2.0-biXDCR-all/303/console
Rebalance completed successfully in this run

2013-04-19 05:19:12 | ERROR | MainProcess | Cluster_Thread | [rest_client._rebalance_progress] unable to reach the host @ 10.3.121.56
2013-04-19 05:21:23 | ERROR | MainProcess | Cluster_Thread | [rest_client._http_request] socket error while connecting to http://10.3.121.56:8091/pools/default/rebalanceProgress error timed out

andrey@baranouski:~/repository/testrunner$ curl -u admin:password 'http://Administrator:Password@10.3.121.156:8091/pools/default/rebalanceProgress'
curl: (7) couldn't connect to host

ps: for both cases UI/clusters hang