Error when importing a project from another GitLab server

I have a fresh GitLab install and when I try to import my old projects via the GitLab export feature, I get this error message:

The repository could not be imported.
undefined method `Error' for Gitlab::ImportExport:Module

If I click on the Start import button anyway, I get a 500.

Bildschirmfoto_vom_2016-08-02_15-18-10

Thanks for any help.