Thanks for your reply.
Yes it is.
I got the Validation extraction results from the action center (activity wait for document validation and resume).
And then it is used in the Train extractors scope.
And ACResults variable is an ExtractionResult variable type.
And as I mentionned before, the weird thing is that it is working most of the time. It is just from time to time (like it happened 3 times over a 100 extraction results) that I got this error.
Do you think it could come from the user while they are validating their data in the action center?