Hey Dave,
I started from scratch, followed the instructions, installed production server 3.0.1, self-signed the certificate, downloaded the RPi OS image with client 3.1, and configured:
root@raspberrypi:~# head -10000 /etc/mender/mender*conf
==> /etc/mender/mender.conf <==
{
    "HttpsClient": {},
    "Security": {},
    "SkipVerify": true,
    "RootfsPartA": "/dev/mmcblk0p2",
    "RootfsPartB": "/dev/mmcblk0p3",
    "DeviceTypeFile": "/var/lib/mender/device_type",
    "DBus": {
        "Enabled": true
    },
    "UpdateControlMapExpirationTimeSeconds": 90,
    "UpdateControlMapBootExpirationTimeSeconds": 45,
    "UpdatePollIntervalSeconds": 5,
    "InventoryPollIntervalSeconds": 5,
    "RetryPollIntervalSeconds": 30,
    "ServerCertificate": "/certificate.crt",
    "Servers": [
        {
            "ServerURL": "https://my-host"
        }
    ]
}
==> /etc/mender/mender-connect.conf <==
{
  "ServerCertificate": "/certificate.crt",
  "SkipVerify": true,
  "User": "pi",
  "ShellCommand": "/bin/bash"
}
(I had to add SKipVerify as I used very short and self-signed certificate).
I was unable to replicate the issue, it works. Here is the docker ps and versions output:
CONTAINER ID        IMAGE                                                COMMAND                  CREATED             STATUS                       PORTS                          NAMES
c3bcbd19d380        mendersoftware/deployments:mender-3.0.1              "/entrypoint.sh --co…"   About an hour ago   Up About an hour             8080/tcp                       menderproduction_mender-deployments_1
92678bd4db5d        traefik:v2.4                                         "/entrypoint.sh --ac…"   About an hour ago   Up About an hour             80/tcp, 0.0.0.0:443->443/tcp   menderproduction_mender-api-gateway_1
399e03cb0564        mendersoftware/deviceauth:mender-3.0.1               "/usr/bin/deviceauth…"   About an hour ago   Up About an hour             8080/tcp                       menderproduction_mender-device-auth_1
c8e7958b5dc7        mendersoftware/deviceconfig:mender-3.0.1             "/usr/bin/deviceconf…"   About an hour ago   Up About an hour             8080/tcp                       menderproduction_mender-deviceconfig_1
f45813bf60bd        mendersoftware/useradm:mender-3.0.1                  "/usr/bin/useradm --…"   About an hour ago   Up About an hour             8080/tcp                       menderproduction_mender-useradm_1
dfcc50b2f366        mendersoftware/workflows:mender-3.0.1                "/usr/bin/workflows …"   About an hour ago   Up About an hour             8080/tcp                       menderproduction_mender-workflows-server_1
58873ef6c61f        mendersoftware/inventory:mender-3.0.1                "/usr/bin/inventory …"   About an hour ago   Up About an hour             8080/tcp                       menderproduction_mender-inventory_1
5ff1a8ead75b        mendersoftware/deviceconnect:mender-3.0.1            "/usr/bin/deviceconn…"   About an hour ago   Up About an hour             8080/tcp                       menderproduction_mender-deviceconnect_1
2696e2c62494        mendersoftware/create-artifact-worker:mender-3.0.1   "/usr/bin/workflows …"   About an hour ago   Up About an hour             8080/tcp                       menderproduction_mender-create-artifact-worker_1
e7acdbd8ff3a        mendersoftware/workflows-worker:mender-3.0.1         "/usr/bin/workflows …"   About an hour ago   Up About an hour                                            menderproduction_mender-workflows-worker_1
02306295e14d        mongo:4.4                                            "docker-entrypoint.s…"   About an hour ago   Up About an hour             27017/tcp                      menderproduction_mender-mongo_1
c11b2f1016ce        mendersoftware/gui:mender-3.0.1                      "/entrypoint.sh nginx"   About an hour ago   Up About an hour (healthy)   80/tcp, 8080/tcp               menderproduction_mender-gui_1
833ccf22fd18        minio/minio:RELEASE.2019-04-23T23-50-36Z             "/usr/bin/docker-ent…"   About an hour ago   Up About an hour (healthy)   9000/tcp                       menderproduction_minio_1
636af8ec4dfb        nats:2.1.9-alpine3.12                                "docker-entrypoint.s…"   About an hour ago   Up About an hour             4222/tcp, 6222/tcp, 8222/tcp   menderproduction_mender-nats_1
client:
root@raspberrypi:~# mender-connect --version
mender-connect version 1.2.0	runtime: go1.14.7
root@raspberrypi:~# mender --version
3.1.0	runtime: go1.14.7
The question now is: what is the difference between our setups?
One request: could you disable IPv6 on all interfaces, restart the dockerd and the server composition, and send me docker ps then?
peter