The issue was found and fixed with Release.Manager 8.0.4 in Release Manager. The Package is available for download at the Automic-Support-Zone (https://support.automic.com).
Hotfix-Description: Error when upgrading a package to a new package type version which creates new milestones between existing milestones.
An issue has been fixed where, when upgrading a package to a new package type version which creates new milestones between existing milestones, an error occurred.
The issue also got fixed in the following version
Release.Manager 10.0.0,
Release.Manager 9.0.2