Gitlab autoscaling: "Failed to request job: runner requestConcurrency meet" - gitlab

Question

We've set up a Gitlab autoscaling master machine as described in the official docs.
The gitlab-runner instance works, is recognized by gitlab.org, and also successfully spawns runners to execute the jobs.
However, the jobs don't get really started on the spawned runners. They stick at this point.
We have debug-level logging turned on, and the only unhappy-looking messages are these repeated ones:
msg="Failed to request job: runner requestConcurrency meet"
config.toml looks like this:
concurrent = 20
check_interval = 10
log_level = "debug"
log_format = "text"
[session_server]
session_timeout = 1800
[[runners]]
name = "gitlab-runner-master"
url = "https://gitlab.com/"
token = "blabla"
executor = "docker+machine"
limit = 25
[runners.custom_build_dir]
[runners.cache]
Type = "s3"
Shared = true
[runners.cache.s3]
ServerAddress = "s3.amazonaws.com"
AccessKey = "blabla"
SecretKey = "blabla"
BucketName = "gitlab.cache.dyynamo.net"
BucketLocation = "us-east-1"
[runners.cache.gcs]
[runners.cache.azure]
[runners.docker]
tls_verify = false
image = "amd64/ubuntu:16.04"
privileged = true
disable_entrypoint_overwrite = false
oom_kill_disable = false
disable_cache = true
volumes = ["/cache"]
shm_size = 0
[runners.machine]
IdleCount = 0
IdleTime = 600
MaxBuilds = 100
MachineDriver = "amazonec2"
MachineName = "gitlab-docker-machine-%s"
Any ideas what the problem can be?
Have also noted this still-open similar issue.