PBS fails to change TMPDIR

Hi,
I am having some trouble to change the default tmpdir from /var/tmp to another location, such as /scratch/tmp (a different hard drive). I try editing either /etc/pbs.conf or mom_priv/config, or both, but none of them works. My pbs.conf:
PBS_SERVER=litio
PBS_START_SERVER=1
PBS_START_SCHED=1
PBS_START_COMM=1
PBS_START_MOM=1
PBS_EXEC=/opt/pbs
PBS_HOME=/var/spool/pbs
PBS_CORE_LIMIT=unlimited
PBS_SCP=/bin/scp
PBS_TMPDIR=/scratch/tmp
Delivers the error:
pbs_mom: Operation not permitted (1) in tmp_file_sec, Security violation "/scratch/tmp" resolves to "/scratch"
/opt/pbs/sbin/pbs_mom: Unable to configure temporary directory.
pbs_mom startup failed, exit 1 aborting.
When executing
sudo /etc/init.d/pbs restart
as stated in section 14.8.2 of the Admin Guide.
Could someone please help?
Thanks a lot.
Alfonso