Gridpack wih Nan x-section (and Nan XERRUP in lhe) for pp -> DY->ll wtih mll>50GeV and 4jets

In the context of CMS I'm trying to generate LO DY->ll with mll>50GeV and up to 4 jets in Madgraph5_aMC@NLO version 2.4.2 .
The gridpack is produced, apparently without errors, however the cross-section is not computed. From the gridpack generation log:

  === Results Summary for run: pilotrun tag: DY ===
     Cross-section : nan +- nan pb
     Nb of events : 0

When using the gridpack to generate events a LHE file is created but the CMS interface to LHE files fails to read it.
Basically the created LHE file has "NaN" entries in the initialization information, see (*).
My understanding is that the "NaN" refers to the XERRUP in the HEPRUP block, but I don't know who is supposed to fill it and when.

--> Do you have suggestion about this issue and how to solve it?
This is a show-stopper for the current official production of this sample in CMS.

The relevant stuff is in a compressed tarball in: https://www.dropbox.com/s/72uk2d34i48gj5p/DYJets-incl-mll50-up4jets.tgz?dl=0
It contains:
 - the gridpack itself and the gridpack generation log
 - the produced lhe file (named cmsgrid_final.lhe) and the log related to generation of events

Thank you very much,
Alessandra

(*)
...
<init>
     2212 2212 0.65000000000E+04 0.65000000000E+04 0 0 306000 306000 3 5
  0.10380820359E+04 NaN 0.00000000000E+00 4
  0.14347071549E+04 NaN 0.00000000000E+00 3
  0.19800505796E+04 NaN 0.00000000000E+00 2
  0.28641569150E+04 NaN 0.00000000000E+00 1
  0.54146115604E+04 NaN 0.00000000000E+00 0
<generator name='MadGraph5_aMC@NLO' version='#5.2.4.2'>please cite 1405.0301 </generator>
</init>
....