Authentication error behind proxy

Hi, i’m new to Docker.
I’m running (or at least try to do so) docker for mac behind a proxy (no ssl interception).
The traffic is forwarded to cntlm.
Starting Docker beta and input my invitation code leads to the following error message:
Authentication error
Put https://validate.docker.com/api: dial tcp 52.4.27.208:443: i/o timeout

curl’ing https://validate.docker.com results in a “Method not allowed” response from the server, so i guess i can reach the URL.
I cannot ping docker.com, maybe this is related to this topic?

Does anybody see this behaviour bevor?
I can’t figure out, why this is not working.