Hello List,
I've finally found the culprit: in addition to libslurm32 from the slurm module, there's a libslurm31 in the chroot. For some reason, this is used when srun starts MPI jobs, and this triggers the error message (libslurm31 is 17.02).
I didn't notice this until I re-compiled Slurm with a custom message. Plus, it's not clear to me why the outdated libslurm.so get used at all.
Anyway, case closed since I can start MPI jobs now.
A.