It worked! My instance and dashboard are back online with all my data restored. I can feel the awesomeness of Cloudron again.

In the end the install with ./cloudron-setup --version 5.6.3 seem to have worked on Ubuntu LTS (after a minor version downgrade of NGINX). Now that I've mastered the install/restore I can always go back to Ubuntu 18 if I have any problem.

Thank you again for your help.