Working on this in parallel and just wanted to dump the status so far.
The reason discourse does not start is the instructions are "git clone" and that checks out the latest plugin code which is sometimes incompatible with the installed discourse version.
https://meta.discourse.org/t/pinning-plugin-and-theme-versions-for-older-discourse-installs/156971 says "You can now include a .discourse-compatibility file at the root of a plugin or theme repository, that designates what to checkout when installing on older Discourse versions."
Very nice, so we have to figure how to checkout the correct version. And Indeed that calendar plugin has moved on to min compat version of 2.9beta of discourse, we are only on 2.8 still.
https://meta.discourse.org/t/migration-failed-on-update-calendar-plugin-to-last-commit/239437 and https://meta.discourse.org/t/cloudron-missing-plugin-pull-compatible-all/244002 suggests there is some plugin:pull_compatible_all rake task. So, we have to figure how to use this in Cloudron.