Error after restarting: particles communicated to PME rank 0 are more than 2/3 times the cut-off

GROMACS version: 2019.4
GROMACS modification: with plumed

Dear all,
My simulation has crashed with the following error:

-------------------------------------------------------
Program:     gmx mdrun, version 2019.4
Source file: src/gromacs/ewald/pme-redistribute.cpp (line 282)
MPI rank:    0 (out of 6)

Fatal error:
789 particles communicated to PME rank 0 are more than 2/3 times the cut-off
out of the domain decomposition cell of their charge group in dimension x.
This usually means that your system is not well equilibrated.

The problem is the simulation has been running for 263400000 steps (526800 ps), after an NVT and NPT equilibration. So I do not think the problem is the system is not well equilibrated.
Curiously the error appeared just after restarting from a checkpoint file. Could this be related to the problem? The run had been restarted several times before without problems.

If so, is there anything I can check or do to solve the error?

Best regards,

Ramon