Cannot import in a group a gitlab-exported project with milestones

Summary

It seems I cannot import into a group a project which I exported from gitlab, if it has project milestones.

Steps to reproduce

1. Create in gitlab a project with issues and project milestones
2. Export the project
3. Create a new group
4. Try to import the project to the same account, in the newly created group

==> I get an error and the project cannot be imported Error importing repository into [project name] - Validation failed: Group milestone should belong either to a project or a group

Example Project

initial project: https://gitlab.com/alexandra.casapu/initial-milestones-project project where I want to import the export of the initial project: https://gitlab.com/group-for-imported-project/imported-project/import/new

What is the current bug behavior?

I get a validation error and the project is not imported.

What is the expected correct behavior?

I should be able to import that project, as I don't have any group milestones defined. I only have project milestones. Even if they have the same name, they should be different accross projects. Maybe I have 2 projects which have the milestone "Version 1 release", but I want to set different dates for the 2 releases, so it makes sense to be able to have different milestones with the same names, in different projects.

Output of checks

I can reproduce the bug on gitlab.com, and also on my local installation (GitLab 9.4.0).

Possible fixes

This might be related to the group milestones feature, but I haven't pinpointed the problem in the code. I have tried a workaround for this, namely to simply import the project in my account, and then to move it to the group, but I get an error: "Cannot move project", without any further details. I have a lot of issues(tasks) and milestones defined and I would not like to spend the time to create them again, so fixing this bug would help me greatly.