In lactose there are lone-pairs (Drudes) on atoms in the linkage between the two residues. These atoms then are from different groups, and this is not handled well in parallel, probably because the atoms may be on different processors.
I would suggest that you try making a single RESI LACT in the rtf, and make sure that all atoms required in the defintion of a lone-pair are in the same group, as defined by the GROUP statments in the RTF. DO NOT MAKE THE WHOLE RESIDUE INTO A SINGLE GROUP. Groups should be no larger than 12Å across.

Lennart Nilsson
Karolinska Institutet
Stockholm, Sweden