Using 'scheduler': 'slurm', an invalid sbatch file is generated with --job-option nice=-1000:
#!/bin/bash
[...]
#SBATCH --nice -1000
ReFrame split the option in two arguments by removing the =, but that's not valid for Slurm as the second argument will begin with a - and is interpreted as another option:
$ srun --nice -1000 true
srun: invalid option -- '1'
Try "srun --help" for more information
One could also argue it's a command-line parsing bug in Slurm, but also I don't think the split on = is documented right now.