As a follow-up to yesterday’s post here, I was able to successfully rebuild my Digital Ocean cloud server keeping the same IP. There were a few hoops to jump through, though. After the restore, running sudo mailinabox crashed at the SSL stage. Basically, I had to delete the /home/user-data/ssl directory then recreate it. Running sudo mailinabox again, it crashed at installing Nextcloud. I had to back up the config.php file in the /home/user-data/owncloud directory then create an empty config file. Ran the mailinabox script again and got Nextcloud to do it’s thing, then had to reverse the config.php backup. Running the script then let Nextcloud upgrade, but left it in maintenance mode. After that everything completed.
I had to log into the admin panel using the IP address and then reprovision all the SSL/TLS certificates. I had to do this several times to get all the certificates provisioned properly. Once that was done, I was able to access the admin panel using the domain name. Then I had to SSH back in and open the owncloud config.php file to reset maintenance mode to false. I ran the sudo mailinabox script one more time and it was smooth sailing. Both Roundcube and Nextcloud work properly now where before both were broken.
I want to thank @alento especially, @openletter and @KiekerJan for their help.