Redmine

• create a new project "test"
• create a child of "test" named "child"
• create a version for the "child" project
• create an issue in "test" and assign it to the "child" version
• go to Administration/Project screen and click the archive link for child
• click "yes" to the "are you sure" dialog

Issue:
The error "This project can not be archived" appears.

If this is the only reason this error can appear than it should be made more descriptive ("This project can not be archived because another project has an issue assigned to one of it's versions" or add documentation on redmine.org which explains this problem.