Import from Bitbucket to Gitlab fails

I'm using gitlab on my Raspberry Pi 2 B using Let's encrypt certificates. I configured the bitbucket oauth as described here (http://stratus3d.com/blog/2015/09/06/migrating-from-bitbucket-to-local-gitlab-server/).

Problem: The first import by an existing repo from bitbucket to gitlab succeeds, but when I try importing another repos, it fails. Another strange thing is, that the "Import all projects" Button is always loading regardless I am importing or not. New: The first import process after gitlab-ctl restart succeeds.

Question: Is there any way to find details about why the import failed in detail? The production.log does not contain something helpful. Why does one import process succeed after restarting gitlab?

Things I tried:

3x redoing the methods exactly described in the tutorial above (incl. new SSH Keys and new API Keys)
changing timeouts in gitlab.rb
CPU and RAM usage is normal and do not exceed the limits
restarting gitlab, restarting server etc. (After restart one import process succeeds)

My Configuration:

Raspberry Pi
    A 900MHz quad-core ARM Cortex-A7 CPU
    1GB RAM
    Raspbian on 64GB USB 3.0 Stick

    Gitlab

        GitLab 8.7.3

        GitLab Shell 2.7.2

        GitLab API v3

        Git 2.7.4

        Ruby 2.1.8p440

        Rails 4.2.6

        PostgreSQL 9.2.15
    Let's encrypt SSL Certificates

Could someone help me, please?