UPDATE: problem solved!
The ARCHER2 Helpdesk got back to me and said they applied a patch to the Lustre filesystem over the past few days.
I tested this with several suites and the postproc now seems to be running fine again on the serial nodes! One additional thing I had to to do was to request a bit more memory for the postproc jobs (the default is just below 2GB). 10 GB works fine for me, but I did not test other values.
If anybody runs into the same OOM error, I just added the memory request to the postproc resources in the archer2.rc
configuration:
[[POSTPROC_RESOURCE]]
inherit = HPC_SERIAL
pre-script = """
module load postproc
module list 2>&1
ulimit -s unlimited
"""
[[[directives]]]
--mem=10G
Best wishes,
Seb