Additional information and possible solution:
I have been trying to figure out why this occurs by varying size of box, number of procs, version of LAMMPS, etc…
I found out that the problem is actually due to SLURM, in particular to the line
#SBATCH --exclusive in the job script.
If I remove it, the problem vanishes and the simulation runs smooth. I tried with different number of procs and box size.
Christophe