Just wanted to share what happened to me and the way to fix it in case you experience the same issue. @cloudron_admin please delete the post if not relevant.

After applying recent Ubuntu security updates and rebooting, my Cloudron instance became unreachable.

cloudron-support --troubleshoot

showed Docker was down, and

systemctl start docker

failed while running dockerd manually worked fine.

I've worked it out with the support of ChatGPT. Root cause : An outdated Cloudron systemd override file:

/etc/systemd/system/docker.service.d/cloudron.conf

This override replaces Docker’s default ExecStart and uses older flags (like cgroupfs) that are no longer compatible with newer Docker versions. As a result, Docker fails to start via systemd.

Fix : Remove the override and reload systemd:

sudo rm /etc/systemd/system/docker.service.d/cloudron.conf
sudo systemctl daemon-reexec
sudo systemctl daemon-reload
sudo systemctl restart docker

Verify Docker is running:

systemctl status docker

Then restart Cloudron:

sudo systemctl restart box

Result

Docker starts normally again, containers come back up, and the Cloudron dashboard becomes accessible.

Notes
Modern Docker versions already handle most of the options that the override was forcing.
Removing the override lets Docker run with its correct defaults (including proper cgroup handling).
This issue may appear after unattended upgrades or manual system updates.

Hope this helps anyone running into the same issue