Snakemake: R script fails (almost) immediately - r

Question

Once more I'm encountering an error in my snakemake workflow that doesn't make any sense to me.
This is the error I get:
[Thu Jan 25 10:47:00 2018] Building DAG of jobs...
[Thu Jan 25 10:47:01 2018] Provided cores: 24
[Thu Jan 25 10:47:01 2018] Rules claiming more threads will be scaled down.
[Thu Jan 25 10:47:01 2018] Job counts:
[Thu Jan 25 10:47:01 2018] count jobs
[Thu Jan 25 10:47:01 2018] 1 merging_seurat
[Thu Jan 25 10:47:01 2018] 1
[Thu Jan 25 10:47:01 2018] Job 0: --- Merging samples using seurat.
Error in setClass("Snakemake", slots = c(input = "list", output = "list", :
unused argument(s) (slots = c(input = "list", output = "list", params = "list", wildcards = "list", threads = "numeric", log = "list", resources = "list", config = "list", rule = "character"))
Execution halted
[Thu Jan 25 10:47:02 2018] Error in rule merging_seurat:
[Thu Jan 25 10:47:02 2018] jobid: 0
[Thu Jan 25 10:47:02 2018] output: merging_seurat/12_top10_heatmap_all_wilcox.pdf, merging_seurat/13_top10_heatmap_all_roc.pdf, merging_seurat/merging_seurat.RData
[Thu Jan 25 10:47:02 2018] RuleException:
[Thu Jan 25 10:47:02 2018] CalledProcessError in line 372 of .../snakemake_pipeline/Snakefile:
[Thu Jan 25 10:47:02 2018] Command ' set -euo pipefail; Rscript .../snakemake_pipeline/scripts/.snakemake.jv8ijpiw.merging_seurat.R ' returned non-zero exit status 1
[Thu Jan 25 10:47:02 2018] File ".../snakemake_pipeline/Snakefile", line 372, in __rule_merging_seurat
[Thu Jan 25 10:47:02 2018] File ".../tools/anaconda3/envs/Seurat/lib/python3.5/concurrent/futures/thread.py", line 55, in run
[Thu Jan 25 10:47:02 2018] Will exit after finishing currently running jobs.
[Thu Jan 25 10:47:02 2018] Exiting because a job execution failed. Look above for error message
[Thu Jan 25 10:47:02 2018] Complete log: .../snakemake_6/.snakemake/log/2018-01-25T104700.498155.snakemake.log
This is the rule in question:
rule merging_seurat:
input: expand("{sample}/molecule_count/counts_wide.tsv", sample=config["samples"]),
output: "merging_seurat/12_top10_heatmap_all_wilcox.pdf",
"merging_seurat/13_top10_heatmap_all_roc.pdf",
"merging_seurat/merging_seurat.RData"
message: "--- Merging samples using seurat."
script: "scripts/merging_seurat.R"
Here we have the top of the R script that is still executed:
sink('merging_seurat/output.txt')
print(installed.packages())
print(sessionInfo())
sink()
And these are the following lines in the R script that are not executed anymore:
library('Seurat')
library('dplyr')
library('org.Hs.eg.db')
Because I logged the packages that are available, I know that the three packages that should be loaded are also installed so it shouldn't fail because of that. In fact, commenting these lines out doesn't change anything, the script still breaks and the log message that should come right after loading the packages is not written to the log file.
Finally, this is the command that I use to run snakemake:
snakemake --use-conda \
--latency-wait 90 \
--rerun-incomplete \
--keep-going \
--timestamp \
--cluster-config SGE.json \
--cluster "qsub -cwd -N {cluster.name} -l h_vmem={cluster.h_vmem},h_stack=256M -o {cluster.stdout}{cluster.name}.o -e {cluster.stderr}{cluster.name}.e -m {cluster.mailtype} -M {cluster.mailuser}" \
-j 8 \
--directory .../snakemake_6
Does anybody have a clue what the error message could mean? unused argument of the snakemake R object is strange because I'm calling multiple parameters later on in the script.
What is even more strange is that I have another R script that works with different packages but has very similar first lines runs fine. I remember that in the beginning I had the same problem with that script (same error message) but don't remember how I solved it.
Any help is well appreciated.