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}]}]