Post Error 400 when using with Synology Surveilliance Station - AI Motion Detection

Hi all,
im using Deepstack latest (and tried cpu-3beta) and christofo/sssai Docker Containers on a Synology DS220+ NAS and folowed this guide: where sssai submits the images to Deepstack and waits for the response.

Every time an image is being sent to Deepstack I get an error 400 in the logs of the Deepstack container:
[GIN] 2021/10/06 - 20:37:11 | 400 | 12.419371ms | | POST "/v1/vision/detection"

but when I send an image with CURL to Deepstack, it works like a charm:
[GIN] 2021/10/06 - 20:56:14 | 200 | 1.583544142s | | POST "/v1/vision/detection"

I already posted this in the blog (see above), but I hope you could point me in the right direction what an error 400 in Deepstack means…