Converge error: Malformed response first line

[tf]

No matter if I run sure-deploy from the docker image or as a manually compiled executable, I always get:

$ sure-deploy converge --host <ip> --port <port> <stack_name>
(monitor.ml.Error
 (Failure "Malformed response first line: \021\003\001\000\002\002")
 ("Raised at file \"pervasives.ml\", line 32, characters 17-33"
  "Called from file \"src/deferred1.ml\", line 20, characters 40-45"
  "Called from file \"src/job_queue.ml\", line 159, characters 6-47"))

$ docker -v
Docker version 18.05.0-ce, build f150324

On Ubuntu 16.04.

Any hints appreciated.