When eZ triggers yoochoose (look 2th point: https://doc.yoochoose.net/display/PUBDOC/Import+V2) wrong header is used: Content-Type 'application/octet-stream' and should be 'application/json'.

Another important things:
1)In body events should be always an array (also when there is 1 item). "contentTypeId" shouldn't be an array - if there is more "contentTypeId's" then each of them should has own event.
2)Header "Authorization: Basic xxx" is missing
3)In body events the "lang" field is missing.
4)Field "transaction" cannot be null