The solution is to just start over with a fresh Ubuntu install and installing Cloudron. Another important thing to remember is to provide the correct version as part of the installation. It must be ./cloudron-setup --provider generic --version <version>. The version field is in the backup id.
@gofaniyi SOGo is a "frontend" to email. The emails itself are stored in the backend which is part of Cloudron. Practically, what this means is that if you install some other email frontend like rainloop or roundcube, you will be able to see the same emails.
To answer your question, all the data will be in /home/yellowtent/boxdata/mail/vmail. You will find directories organized by domain.com/username. They are all in maildir format.
Actually, the next time, if you run any linux distribution on your pc, then I would recommend to install imapsync there and do the one-time syncing, this avoids having to install additional packages alongside of Cloudron on that server.
Cloudron will "take over" the server and also manages the packages and services, which are installed. Generally it is not supported to install anything on the side. In this case Cloudron depends on mysql-server-5.7 which will probably uninstall mariadb through apt.
the application's data path can be found at /home/yellowtent/appsdata/<appId>/data/ and that is the only writeable path for apps (besides /tmp and /run of course). This is done for security as well as predictability for updates. Usually if you push data there through external means like SSH, you might have to reconfigure the app through the dashboard, in case the file permissions are not correct. Generally it is preferred that data changes are done via the app itself, which I guess is not quite applicable for your use-case though.
For duplicator, from what I understand this migration script is mostly just creating a tarball/zipfile of the whole previous installation and then extracts it into a folder, which is served up by some LAMP environment. This is incompatible with the Cloudron WordPress package, for reasons mentioned above (mostly the read-only fashion) For this you probably have to fallback to the LAMP app itself. There it should work.