It appears that Server 5.1 doesn’t properly activate webapps for the two default websites.
This issue only effects roundcube installations upgraded to Server 5.1
Roundcube can be installed or re-installed on Server 5.1 without issue.
To re-activate the webapp after upgrading to Server 5.1, run this command:
sudo /Applications/Server.app/Contents/ServerRoot/usr/sbin/webappctl start com.topicdesk.roundcube