HI Federico and Benjamin,
I have been looking at this today. I think there are two parts to this issue, 1) It's not clear to me that the request will accept a JSON body as documented, so we need to look at that. And 2) if we use the workaround and send a more standard POST body, e.g.:
We see another issue appear where Archivematica may be looking in the wrong directory to try and initiate the re-ingest.
Let us know how it goes though and what you'd like to achieve.
Regarding an Archivematica/API Client then you might also be interested in the Automation Tools work here:
https://github.com/artefactual/automation-tools#archivematica-client,
amclient.py wraps a lot of the API calls up to be a little more user friendly to use. We make use of it as a module too in places, but it might prove to have some useful command line functionality for you.
I hope that helps,
Ross