Hi @Sigve-opedal_6476 ,
No idea why it wouldn't work, but I would look at something like ProxyMan or Wireshark to capture HTTP traffic, and see what requests are different.
You should see a pattern of requests that work, and a pattern that doesn't.
Maybe the client is requesting some other file that you aren't uploading? I don't think there's an API or HTTP header... I think it's all basic GET requests. But that will tell you the delta.
Thanks,
Alana