How to relate issues that belong to different projects in Redmine
Redmine can relate two issues that are connected to each other. With the default settings, you can relate only issues that belong to the same project.
To set a relation to an issue in another project, enable the setting "Allow cross-project issue relations" on the "Issue tracking" tab of the "Administration" → "Settings" page.
If you try to relate an issue in another project while "Allow cross-project issue relations" is disabled, the error "doesn't belong to the same project" is displayed.

