Importing an exported project fails with "wrong argument type nil (expected String)"

Summary

We need to move some projects from one GitLab server to another GitLab server (both are version 9.4.4-ee). We export the projects and import them on the other server. The export works as expected, but the import fails for some projects.

Steps to reproduce

  1. Export an existing projects (Settings -> Export project) and download the *.tar.gz
  2. Create a new project on another server and choose "Import project from: GitLab export"
  3. Choose the previously downloaded *.tar.gz and click on "Import project"

Example Project

Unfortunately none (cannot upload our projects).

What is the current bug behavior?

After clicking on "Import project", the following error appears:

The repository could not be imported.
Error importing repository  into <PROJECTPATH> - wrong argument type nil (expected String)

After that, the new project was created, but it does not contain any data (not even the git repository).

What is the expected correct behavior?

The project is imported successfully and contains all data as at the time the export was created.

Relevant logs and/or screenshots

gitlab_import_error_2

(I don't know why the form for importing a Git repository appears here, I started a GitLab-export import, not a Git import.)

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info 
System information
System:		Ubuntu 16.04
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.3.3p222
Gem Version:	2.6.6
Bundler Version:1.13.7
Rake Version:	10.5.0
Redis Version:	3.2.5
Git Version:	2.13.4
Sidekiq Version:5.0.0
Go Version:	unknown

GitLab information
Version:	9.4.4-ee
Revision:	c7bc62c
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
DB Version:	9.6.3
URL:		https://(REMOVED)
HTTP Clone URL:	https://(REMOVED)/some-group/some-project.git
SSH Clone URL:	git@(REMOVED):some-group/some-project.git
Elasticsearch:	no
Geo:		no
Using LDAP:	yes
Using Omniauth:	no


GitLab Shell
Version:	5.3.1
Repository storage paths:
  • default: /var/opt/gitlab/git-data/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check 
...
Checking GitLab ... Finished

Possible fixes

Don't know. But it seems that the error appears only on projects which have merge requests and/or issues.