Problems running MG using CKKW-L matching scheme

Dear MG developers,

I'm generating events for the ttbar + jets process and whereas the parton-level part works the parton shower gets stuck. The MG version is the latest.

The prototype of the pythia8 card being used is this:

!
! Pythia8 cmd card automatically generated by MadGraph5_aMC@NLO
! For more information on the use of the MG5aMC / Pythia8 interface, visit
! https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/LOPY8Merging
!
! ==================
! General parameters
! ==================
!
Main:numberOfEvents = number
!
! -------------------------------------------------------------------
! Specify the HEPMC output of the Pythia8 shower. You can set it to:
! auto : MG5aMC will automatically place it the run_<i> directory
! /dev/null : to turn off the HEPMC output.
! <path> : to select where the HEPMC file must written. It will
! therefore not be placed in the run_<i> directory. The
! specified path, if not absolute, will be relative to
! the Event/run_<i> directory of the process output.
! fifo : to have MG5aMC setup the piping of the PY8 output to
! analysis tools such as MadAnalysis5.
! fifo@<fifo_path> :
! Same as 'fifo', but selecting a custom path to create the
! fifo pipe. (useful to select a mounted drive that supports
! fifo). Note that the fifo file extension *must* be '.hepmc.fifo'.
! -------------------------------------------------------------------
!
HEPMCoutput:file = events.hepmc
!
! --------------------------------------------------------------------
! Parameters relevant only when performing MLM merging, which can be
! turned on by setting ickkw to '1' in the run_card and chosing a
! positive value for the parameter xqcut.
! For details, see section 'Jet Matching' on the left-hand menu of
! http://home.thep.lu.se/~torbjorn/pythia81html/Welcome.html
! --------------------------------------------------------------------
! If equal to -1.0, MadGraph5_aMC@NLO will set it automatically based
! on the parameter 'xqcut' of the run_card.dat
! The following parameter was forced to be commented out by MG5aMC.
! JetMatching:qCut = -1.0
! Use default kt-MLM to match parton level jets to those produced by the
! shower. But the other Shower-kt scheme is available too with this option.
! The following parameter was forced to be commented out by MG5aMC.
! JetMatching:doShowerKt = off
! A value of -1 means that it is automatically guessed by MadGraph.
! It is however always safer to explicitly set it.
! The following parameter was forced to be commented out by MG5aMC.
! JetMatching:nJetMax = -1
!
! --------------------------------------------------------------------
! Parameters relevant only when performing CKKW-L merging, which can
! be turned on by setting the parameter 'ptlund' *or* 'ktdurham' to
! a positive value.
! For details, see section 'CKKW-L Merging' on the left-hand menu of
! http://home.thep.lu.se/~torbjorn/pythia81html/Welcome.html
! --------------------------------------------------------------------
! Central merging scale values you want to be used.
! If equal to -1.0, then MadGraph5_aMC@NLO will set this automatically
! based on the parameter 'ktdurham' of the run_card.dat
Merging:TMS = 5.0000000000e+01
! This must be set manually, according to Pythia8 directives.
! An example of possible value is 'pp>LEPTONS,NEUTRINOS'
! Alternatively, from Pythia v8.223 onwards, the value 'guess' can be
! used to instruct Pythia to guess the hard process. The guess would mean
! that all particles apart from light partons will be considered as a part
! of the hard process. This guess is prone to errors if the desired hard
! process is complicated (i.e. contains light partons). The user should
! then be wary of suspicious error messages in the Pythia log file.
Merging:Process = pp>tt~
! A value of -1 means that it is automatically guessed by MadGraph.
! It is however always safer to explicitly set it.
Merging:nJetMax = 2
!
! For all merging schemes, decide whehter you want the merging scale
! variation computed for only the central weights or all other
! PDF and scale variation weights as well
SysCalc:fullCutVariation = off
!
! ==========================
! User customized parameters
! ==========================
!
! By default, Pythia8 generates multi-parton interaction events. This is
! often irrelevant for phenomenology and very slow. You can turn this
! feature off by uncommenting the line below if so desired.
!partonlevel:mpi = off
!
! Additional technical parameters set by MG5_aMC.
!
SpaceShower:pTmaxMatch=1
SpaceShower:rapidityOrder=off
SysCalc:tmsList=75.000,100.000
! Value of the merging scale below which one does not even write the HepMC event.
SysCalc:qWeed=5.0000000000e+01
! Tell Pythia8 that an LHEF input is used.
Beams:frameType=4
TimeShower:pTmaxMatch=1
! Do veto externally (e.g. in SysCalc).
Merging:applyVeto=off
! If turned off, then the option belows forces PY8 to keep the original weight.
Merging:includeWeightInXsection=off
Merging:nQuarksMerge=5
! 1.0 corresponds to HEPMC weight given in [mb]. We choose here the [pb] normalization.
HEPMCoutput:scaling=2.0900000000e+07
! Specify one must read inputs from the MadGraph banner.
JetMatching:setMad=off
JetMatching:etaJetMax=1.0000000000e+03
! Be more forgiving with momentum mismatches.
Check:epTolErr=1.0000000000e-02
!
! ====================
! Subrun definitions
! ====================
!
LHEFInputs:nSubruns=1
Main:subrun=0
!
! Definition of subrun 0
!
Beams:LHEF=events.lhe.gz
Merging:doKTMerging=on
Merging:Dparameter=4.0000000000e-01

I have seen that turning "Merging:applyVeto to on" it works.

Besides I have seen this also happens for other processes.

Any clueabout what is going on ?

Best,
Roberto