The installation of puppet-master-passenger fails when installed at the same time puppetdb is installed. I found a workaround by installing puppetdb first, followed by puppet-master-passenger. This installation conflict never occurred in the past when I installed puppetmaster-passenger and puppetdb at the same time on Ubuntu 16.04. The software mirror I used for successful installation was obtained from apt.puppetlabs.com/puppetlabs-release-pc1-xenial.deb. This installation conflict started with the new puppet-master-passenger package obtained from apt.puppetlabs.com/puppet6-release-bionic.deb, therefore this conflict started after puppet-master-passenger was updated for Ubuntu 18.04 as denoted in the changelog.
I tried to contact <email address hidden> who is the last developer to patch puppet-master-passenger according to the changelog, and the email bounced since this email address no longer exists.