Rolling upgrade from 172 to latest 181 fails with failed rebalance {type,exit}, {what,{noproc, {gen_fsm,sync_send_event,}}}

Failing test
upgradetests.MultipleNodeUpgradeTests.test_upgrade,initial_version=1.7.2,create_buckets=True,insert_data=True,start_upgraded_first=False,load_ratio=10,online_upgrade=True

2012-04-18 05:05:23,369 - root - INFO - adding node : 10.3.121.92:8091 to the cluster
2012-04-18 05:05:23,370 - root - INFO - adding remote node : 10.3.121.92 to this cluster @ : 10.3.121.98
2012-04-18 05:05:24,121 - root - INFO - added node : ns_1@10.3.121.92 to the cluster
2012-04-18 05:05:24,134 - root - INFO - rebalance params : password=password&ejectedNodes=&user=Administrator&knownNodes=ns_1%4010.3.121.94%2Cns_1%4010.3.121.92%2Cns_1%4010.3.121.98%2Cns_1%4010.3.121.93%2Cns_1%4010.3.121.97%2Cns_1%4010.3.121.95
2012-04-18 05:05:24,140 - root - ERROR - http://10.3.121.98:8091/controller/rebalance error 500 reason: unknown ["Unexpected server error, request logged."]
2012-04-18 05:05:24,140 - root - ERROR - rebalance operation failed

INFO REPORT <0.6440.0> 2012-04-18 05:06:53
===============================================================================

ns_log: logging menelaus_web:19:Server error during processing: ["web request failed",

{path,"/controller/rebalance"}

,

{type,exit}

,
{what,
{noproc,
{gen_fsm,sync_send_event,
[

{global,ns_orchestrator}

,

{start_rebalance, ['ns_1@10.3.121.94','ns_1@10.3.121.92', 'ns_1@10.3.121.98','ns_1@10.3.121.93', 'ns_1@10.3.121.97','ns_1@10.3.121.95'], [],[]}

]}}},
{trace,
[

{gen_fsm,sync_send_event,2}

,

{menelaus_web,do_handle_rebalance,3}

,

{menelaus_web,loop,3}

,

{mochiweb_http,headers,5}

,

{proc_lib,init_p_do_apply,3}

]}]