You mean that created the task? I probably should have added to the problem description that I'm able to delete the task manually, also using the SDK.
I'll elaborate on the setup.
I'm deploying server in the recommended way with very minor changes. The relevant portions of the yamls:

 agent-services:
    networks:
      - backend
    container_name: clearml-agent-services
    image: allegroai/clearml-agent-services:latest
    deploy:
      restart_policy:
        condition: on-failure
    privileged: true
    environment:
      CLEARML_HOST_IP: ${CLEARML_HOST_IP}
      CLEARML_WEB_HOST: ${CLEARML_WEB_HOST:-}
      CLEARML_API_HOST: 

      CLEARML_FILES_HOST: ${CLEARML_FILES_HOST:-}
      CLEARML_API_ACCESS_KEY: ${CLEARML_AGENT_ACCESS_KEY:-$CLEARML_API_ACCESS_KEY}
      CLEARML_API_SECRET_KEY: ${CLEARML_AGENT_SECRET_KEY:-$CLEARML_API_SECRET_KEY}
      CLEARML_AGENT_GIT_USER: ${CLEARML_AGENT_GIT_USER}
      CLEARML_AGENT_GIT_PASS: ${CLEARML_AGENT_GIT_PASS}
      CLEARML_AGENT_UPDATE_VERSION: ${CLEARML_AGENT_UPDATE_VERSION:->=0.17.0}
      CLEARML_AGENT_DEFAULT_BASE_DOCKER: "ubuntu:18.04"
      AWS_ACCESS_KEY_ID: ${AWS_ACCESS_KEY_ID:-}
      AWS_SECRET_ACCESS_KEY: ${AWS_SECRET_ACCESS_KEY:-}
      AWS_DEFAULT_REGION: ${AWS_DEFAULT_REGION:-}
      AZURE_STORAGE_ACCOUNT: ${AZURE_STORAGE_ACCOUNT:-}
      AZURE_STORAGE_KEY: ${AZURE_STORAGE_KEY:-}
      GOOGLE_APPLICATION_CREDENTIALS: ${GOOGLE_APPLICATION_CREDENTIALS:-}
      CLEARML_WORKER_ID: "clearml-services"
      CLEARML_AGENT_DOCKER_HOST_MOUNT: "/opt/clearml/agent:/root/.clearml"
      SHUTDOWN_IF_NO_ACCESS_KEY: 1
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - /opt/clearml/agent:/root/.clearml
    depends_on:
      - apiserver
    entrypoint: >
      bash -c "curl --retry 10 --retry-delay 10 --retry-connrefused '

' && /usr/agent/entrypoint.sh"

Then I extend this file with this definition to add a fileshare:

 agent-services:
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - /opt/clearml/agent:/root/.clearml
      - /mnt/models/clearml_fileserver:/mnt/fileserver

The associated env file looks like this:

CLEARML_AGENT_GIT_USER=michel.kok@domain.com
CLEARML_AGENT_GIT_PASS=#secretpass
CLEARML_AGENT_ACCESS_KEY=KS6Z0UYO139dsf9KWT9NVCNBGUCZIQ
CLEARML_AGENT_SECRET_KEY=ujnWt81euiPcuFpMZxG_XrpFG5sw_dfa-Fv6oOw9-Vy4-bdnGdYqQtpGoDlprkucQ18
CLEARML_HOST_IP=

CLEARML_FILES_HOST=

CLEARML_API_HOST=

CLEARML_API_ACCESS_KEY=B0HTQFOXJMSOGRY6WfJUP73R1EA6SS
CLEARML_API_SECRET_KEY=uzaaIaEdAVH2Wf-ZLAC6zGoUNFuuDRyfdawS4fgiBr_GkULkxd3Ow15sDNGo3KOWnGc

I'm assuming the agent service runs an up-to-date version of the SDK automatically, shouldn't it?