BackUp Error: pmm utility 'PMMCli' raised an exception. Error code is: -1

[JuanMD]

SO: CentOS 6.4 (Final)
Plesk Panel 11.5.30

I have scheduled one daily BackUp Data "Configuration and Content"

I have received 17 emails this night (from 03:15 to 07:14) with the following error:

Error: pmm utility 'PMMCli' raised an exception. Error code is: -1
See pmmcli.log to find out detailed information on this

Finally the attempt number 18 (07:30) finished the backup.

How can i fix this problem?
Thank you.

For your information, the pmmcli.log is the following:

[2013-09-14 07:14:02.916|8680] DEBUG: LOG: custom log /usr/local/psa/PMM/logs/backup-2013-09-14-07-14-02-013/backup.log
[2013-09-14 07:14:02.922|8680] INFO: Executing <subprocess[8681] '/usr/bin/perl /usr/local/psa/admin/bin/plesk_agent_manager domains-id --get-size --session-path=/usr/local/psa/PMM/sessions/2013-09-14-071402.836 1'>
[2013-09-14 07:14:03.642|8680] INFO: Subprocess raised ExecuteException: Subprocess <subprocess[8681] '/usr/bin/perl /usr/local/psa/admin/bin/plesk_agent_manager domains-id --get-size --session-path=/usr/local/psa/PMM/sessions/2013-09-14-071402.836 1'> was finished with exit code 1
== STDOUT ====================

== STDERR ====================

[2013-09-14 07:14:03.647|8680] CRITICAL: PMMUtility exception:
Subprocess <subprocess[8681] '/usr/bin/perl /usr/local/psa/admin/bin/plesk_agent_manager domains-id --get-size --session-path=/usr/local/psa/PMM/sessions/2013-09-14-071402.836 1'> was finished with exit code 1
== STDOUT ====================

== STDERR ====================

Traceback (most recent call last):
File "/usr/local/psa/admin/share/pmmcli/pmmcli.py", line 2491, in main
data_action_response, errcode_response, error_message = actions.get(sys.argv[1][2:])(parameters)
File "/usr/local/psa/admin/share/pmmcli/pmmcli.py", line 2354, in make_dump
return ActionRunner(MakeDumpAction, backup_task_description, None).doActivity()
File "/usr/local/psa/admin/share/pmmcli/pmmcli.py", line 164, in doActivity
result = self.processor.doActivity()
File "/usr/local/psa/admin/share/pmmcli/pmmcli.py", line 1349, in doActivity
raise PMMUtilityException('PMMCli', x)
PMMUtilityException: PMMCli was finished with exit code 1

[2013-09-14 07:14:03.648|8680] INFO: Outgoing packet:
<?xml version="1.0" encoding="UTF-8"?>
<response>
<errcode>1000</errcode>
<errmsg>pmm utility 'PMMCli' raised an exception. Error code is: 1
See pmmcli.log to find out detailed information on this</errmsg>
</response>