Connection to gitlab fails, update repository dialog allways returns with blank page

I try to connect to a gitlab server and want to get all change sets from the project in gitlab.
I installed the plugins and can't see any problems during installation there.

I open the "Repositories" dialog and put "myrepo" and "gitlab"
In the following dialog I enter:

Name: myrepo
Type: GitLab ( fixed )
URL:  http://localhost:8080
GitLab Root: http://localhost:8080
GitLab Repository ID: ???
GitLab Repository Name: root/Projekt2
GitLab API Key: xxxxxxxxxxxxx
Primary Branches: master

The API-Key I entered I got from my account on gitlab from the page: http://localhost:8080/profile/account.
Is this OK?

If I leave the field "GitLab Repository ID:" empty, and press "Update Repository" and then I only got a blank page! URL: https://localhost/mantis/plugin.php?page=Source/repo_update.php.

If I put some id in ( I don't know how to get the id! ), the following page appears: https://localhost/mantis/plugin.php?page=Source/repo_manage_page&id=10

I have no idea where to find any debug information! Is there any logfile ( in mantis or in gitlab ) to find what is going wrong?
Is it a bug that an empty "GitLab Repository ID" will result in a blank page?

Maybe I filled some invalid parameters to the dialog, so I summarize the gitlab things here:
I can connect to gitlab via: http://localhost:8080/
Cloning from http://localhost:8080/root/Projekt2.git works




mantis version:  	1.2.19
schema:  	183
Plugins:
Source Control Integration 0.18
GitLab Integration 1.0.2

Regards,
 Klaus

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