problem while generating 1M of event while generation of 0.5M works

I'm trying to generate 1M of events for user defined model (throught FeynRules interface) on a PBS cluster. The problem is that everything works if I set the number of events to 0.5M, while it fails for a 1M sample. I'm attaching an output from the cluster. I cut out parts that get repeated over & over and do not give any additional information (they are denoted by ...). The first part gives log from generation of a 1M of events that ends with an error
IOError : [Errno 2] No such file or directory: '/home/2/wkotlarski/sgluons_with_tau/SubProcesses/P0_qq_xxc_x_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq_xc_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq/G1f/results.dat'
I've checked that this file is indeed not present. While, as you can see from the last part of the log, when I set the number of events to 0.5M it finishes ok. What you can also see is that although I requested 0.5M events only 388709 got unweighted. I'm using MadGraph5 v 1.5.1.

No module named madgraph.interface.extended_cmd
No module named madgraph
No module named madgraph
************************************************************
* *
* W E L C O M E to M A D G R A P H 5 *
* M A D E V E N T *
* *
* * * *
* * * * * *
* * * * * 5 * * * * *
* * * * * *
* * * *
* *
* VERSION 5.1.5.1 *
* *
* The MadGraph Development Team - Please visit us at *
* https://server06.fynu.ucl.ac.be/projects/madgraph *
* *
* Type 'help' for in-line help. *
* *
************************************************************
load configuration from /home/2/wkotlarski/sgluons_with_tau/Cards/me5_configuration.txt
load configuration from /home/2/wkotlarski/sgluons_with_tau/Cards/me5_configuration.txt
Using default eps viewer "evince". Set another one in ./input/mg5_configuration.txt
Using default web browser "firefox". Set another one in ./input/mg5_configuration.txt
generate_events -f 7TeV_600GeV --cluster
Will run in mode parton
Generating 1000000 events with run name 7TeV_600GeV
survey 7TeV_600GeV
compile directory
/home/2/wkotlarski/sgluons_with_tau/bin/internal/ufomodel/particles.py
write ./param_card.dat
Using random number seed offset = 21
Running Survey
Creating Jobs
Working on SubProcesses
    P0_gg_xxc_x_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq_xc_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq
 ...
 ...
 ...
    P0_qq_xxc_x_ttx_t_wpb_wp_vltap_tx_wmbx_wm_vltam_xc_ttx_t_wpb_wp_vltap_tx_wmbx_wm_vltam
 Idle: 4 Running: 5 Finish: 219
 Idle: 0 Running: 3 Finish: 225
 Idle: 0 Running: 1 Finish: 227
 Idle: 0 Running: 1 Finish: 227
 Idle: 0 Running: 1 Finish: 227
 Idle: 0 Running: 1 Finish: 227
 Idle: 0 Running: 1 Finish: 227
INFO: All jobs finished
End survey
refine 1000000
compile directory
Creating Jobs
Refine results to 1000000
    P0_gg_xxc_x_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq_xc_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq
 ...
 ...
 ...
    P0_qq_xxc_x_ttx_t_wpb_wp_vltap_tx_wmbx_wm_vltam_xc_ttx_t_wpb_wp_vltap_tx_wmbx_wm_vltam
 Idle: 890 Running: 281 Finish: 15
 ...
 ...
 ...
 Idle: 0 Running: 1 Finish: 1185
INFO: All jobs finished
Combining runs
Start waiting for update on filesystem. (more info in debug mode)
Command "generate_events -f 7TeV_600GeV --cluster" interrupted with error:
IOError : [Errno 2] No such file or directory: '/home/2/wkotlarski/sgluons_with_tau/SubProcesses/P0_qq_xxc_x_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq_xc_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq/G1f/results.dat'
Please report this bug on https://bugs.launchpad.net/madgraph5
More information is found in '/home/2/wkotlarski/sgluons_with_tau/7TeV_600GeV_tag_1_debug.log'.
Please attach this file to your report.
quit

No module named madgraph.interface.extended_cmd
No module named madgraph
No module named madgraph
************************************************************
* *
* W E L C O M E to M A D G R A P H 5 *
* M A D E V E N T *
* *
* * * *
* * * * * *
* * * * * 5 * * * * *
* * * * * *
* * * *
* *
* VERSION 5.1.5.1 *
* *
* The MadGraph Development Team - Please visit us at *
* https://server06.fynu.ucl.ac.be/projects/madgraph *
* *
* Type 'help' for in-line help. *
* *
************************************************************
load configuration from /home/2/wkotlarski/sgluons_with_tau/Cards/me5_configuration.txt
load configuration from /home/2/wkotlarski/sgluons_with_tau/Cards/me5_configuration.txt
Using default eps viewer "evince". Set another one in ./input/mg5_configuration.txt
Using default web browser "firefox". Set another one in ./input/mg5_configuration.txt
generate_events -f 7TeV_600GeV --cluster
Will run in mode parton
Generating 500000 events with run name 7TeV_600GeV
survey 7TeV_600GeV
compile directory
Using random number seed offset = 27
Running Survey
Creating Jobs
Working on SubProcesses
    P0_gg_xxc_x_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq_xc_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq
 ...
 ...
 ...
    P0_qq_xxc_x_ttx_t_wpb_wp_vltap_tx_wmbx_wm_vltam_xc_ttx_t_wpb_wp_vltap_tx_wmbx_wm_vltam
 Idle: 24 Running: 49 Finish: 155
 ...
 ...
 ...
 Idle: 0 Running: 1 Finish: 227
INFO: All jobs finished
End survey
refine 500000
compile directory
Creating Jobs
Refine results to 500000
    P0_gg_xxc_x_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq_xc_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq
 ...
 ...
 ...
    P0_qq_xxc_x_ttx_t_wpb_wp_vltap_tx_wmbx_wm_vltam_xc_ttx_t_wpb_wp_vltap_tx_wmbx_wm_vltam
 Idle: 447 Running: 196 Finish: 19
 ...
 ...
 ...
 Idle: 0 Running: 1 Finish: 661
INFO: All jobs finished
Combining runs
finish refine
refine 500000
compile directory
Creating Jobs
Refine results to 500000
    P0_gg_xxc_x_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq_xc_ttx_t_wpb_wp_qq_tx_wmbx_wm_qq
 ...
 ...
 ...
    P0_qq_xxc_x_ttx_t_wpb_wp_vltap_tx_wmbx_wm_vltam_xc_ttx_t_wpb_wp_vltap_tx_wmbx_wm_vltam
 Idle: 52 Running: 0 Finish: 0
 ...
 ...
 ...
 Idle: 0 Running: 1 Finish: 51
INFO: All jobs finished
Combining runs
finish refine
combine_events
Combining Events
  === Results Summary for run: 7TeV_600GeV tag: tag_1 ===

     Cross-section : 0.08392 +- 2.729e-05 pb
     Nb of events : 388709

store_events
Storing parton level results
End Parton
quit