This 400 error seems to be caused by the "additionalFields" parameter (the API call does not fail anymore if we remove this param).
We where requesting activity.actionObject.tags and activity.actionObject.combinedObjectTypeId as parameters. I can't find any information anywhere about the deprecation of this parameter or about the additionalFields values we are requesting.
Once again, our code did not change and was working well until June 27th 2017.
Does anyone has observed the same behavior or has informations about this topic ?