That did help as did disabling spamassassin on the originating host, but now I'm getting this and the entire migration fails...
31875: 2009-12-16 06:50:02,848 INFO Subprocess raised ExecuteException: Subprocess <subprocess[31877] '/usr/local/psa/admin/bin/launchpad --dump-selected --host=xxx --login=root --session-path=/usr/local/psa/PMM/msessions/2009121606450867 --selected-objects-file=/usr/local/psa/PMM/msessions/2009121606450867/selected_objects --agent-name=PleskX --agent-dir=/migration/2009121606450867/'> was finished with exit code 2
== STDOUT ====================
== STDERR ====================
Error during dump retrieval
31875: 2009-12-16 06:50:02,849 DEBUG Backup status saved to session
31875: 2009-12-16 06:50:02,852 CRITICAL Runtime error in migration handler:
subproc.NonzeroExitException  was finished with exit code 2
Traceback (most recent call last):
  File "/usr/local/psa/admin/share/migration-handler/migration_handler.py", line 1394, in main
    data_action_response, errcode_response, error_message = actions.get(sys.argv[1][2:])(parameters)
  File "/usr/local/psa/admin/share/migration-handler/migration_handler.py", line 1294, in migration_start
    return MigrationHandlerActionRunner(MigrationStartAction, None, migration_session_id).doActivity()
  File "/usr/local/psa/admin/share/migration-handler/migration_handler.py", line 88, in doActivity
    result = self.processor.doActivity()
  File "/usr/local/psa/admin/share/migration-handler/migration_handler.py", line 864, in doActivity
    service.backup(migration_session.getValue('agent_name'), migration_session.getValue('agent_dir'))
  File "/usr/local/psa/admin/share/migration-handler/pmm_migr_remote_service.py", line 95, in backup
    res = self.backupImpl(agent_name, agent_dir)
  File "/usr/local/psa/admin/share/migration-handler/pmm_migr_remote_service_impl.py", line 62, in backupImpl
    self.runCmdLine(cmd)
  File "/usr/local/psa/admin/share/migration-handler/pmm_migr_remote_service.py", line 51, in runCmdLine
    self.session.runWorker(cmd)
  File "/usr/local/psa/admin/share/migration-handler/migration_handler_session.py", line 290, in runWorker
    cmd.spawn()
  File "/usr/local/psa/admin/lib/python/subproc.py", line 240, in spawn
    self.proc.run()
  File "/usr/local/psa/admin/lib/python/subproc.py", line 185, in run
    BaseSubprocess.run(self)
  File "/usr/local/psa/admin/lib/python/subproc.py", line 169, in run
    self.wait()
  File "/usr/local/psa/admin/lib/python/subproc.py", line 193, in wait
    BaseSubprocess.wait(self)
  File "/usr/local/psa/admin/lib/python/subproc.py", line 179, in wait
    raise NonzeroExitException(self, self.__retcode)
NonzeroExitException:  was finished with exit code 2
31875: 2009-12-16 06:50:02,853 INFO Outgoing packet:
<?xml version="1.0" encoding="UTF-8"?>
<response>
    <errcode>1001</errcode>
    <errmsg>Runtime error in migration handler:
subproc.NonzeroExitException  was finished with exit code 2
Traceback (most recent call last):
  File "/usr/local/psa/admin/share/migration-handler/migration_handler.py", line 1394, in main
    data_action_response, errcode_response, error_message = actions.get(sys.argv[1][2:])(parameters)
  File "/usr/local/psa/admin/share/migration-handler/migration_handler.py", line 1294, in migration_start
    return MigrationHandlerActionRunner(MigrationStartAction, None, migration_session_id).doActivity()
  File "/usr/local/psa/admin/share/migration-handler/migration_handler.py", line 88, in doActivity
    result = self.processor.doActivity()
  File "/usr/local/psa/admin/share/migration-handler/migration_handler.py", line 864, in doActivity
    service.backup(migration_session.getValue('agent_name'), migration_session.getValue('agent_dir'))
  File "/usr/local/psa/admin/share/migration-handler/pmm_migr_remote_service.py", line 95, in backup
    res = self.backupImpl(agent_name, agent_dir)
  File "/usr/local/psa/admin/share/migration-handler/pmm_migr_remote_service_impl.py", line 62, in backupImpl
    self.runCmdLine(cmd)
  File "/usr/local/psa/admin/share/migration-handler/pmm_migr_remote_service.py", line 51, in runCmdLine
    self.session.runWorker(cmd)
  File "/usr/local/psa/admin/share/migration-handler/migration_handler_session.py", line 290, in runWorker
    cmd.spawn()
  File "/usr/local/psa/admin/lib/python/subproc.py", line 240, in spawn
    self.proc.run()
  File "/usr/local/psa/admin/lib/python/subproc.py", line 185, in run
    BaseSubprocess.run(self)
  File "/usr/local/psa/admin/lib/python/subproc.py", line 169, in run
    self.wait()
  File "/usr/local/psa/admin/lib/python/subproc.py", line 193, in wait
    BaseSubprocess.wait(self)
  File "/usr/local/psa/admin/lib/python/subproc.py", line 179, in wait
    raise NonzeroExitException(self, self.__retcode)
NonzeroExitException:  was finished with exit code 2
</errmsg>
</response>