InternalFailureException when trying to load turtle files from S3

Hello,

I'm stuck with a problem
I try to upload public turtle data in Neptune and I constantly get the same error :
{"requestId":"xxxxx","code":"InternalFailureException","detailedMessage":"Failed to start load from the source s3://pubchem-rdf-data/data/gene/pc_gene.ttl"}

My data are in a S3 bucket s3://pubchem-rdf-data/data/gene/pc_gene.ttl
My Neptune cluster is up and running
I've setup a role for Neptune named "PubChem-NeptuneLoaderRole", first, with S3readOnly and now with S3FullAcess (in a desperate move). This role is attached to the Neptune cluster
I have setup the S3 endpoint on my VPC, "Full access policy" too

Here is the curl I execute

curl -X POST \
-H 'Content-Type: application/json' \
https://chimie-rdf-store.cluster-ck0rf6wslxzi.eu-west-3.neptune.amazonaws.com:8182/loader -d
{
"source" : "s3://pubchem-rdf-data/data/gene/pc_gene.ttl",
"format" : "turtle",
"iamRoleArn" : "arn:aws:iam::xxxxxxx:role/PubChem-NeptuneLoaderRole",
"region" : "eu-west-3",
"failOnError" : "TRUE",
"mode" : "RESUME",
"parallelism" : "HIGH"
}'

Really, I do not see what I miss / I've done wrong
It seems that the loader cannot access the S3 file (error is almost immediate) but i do not get the standard S3 errors
Any idea ?
Is there any way to get extra information on the error ?

Thanks guys