GitLab clone with GitHub Desktop via HTTPS problem

i just installed GitLab, configured SSL and enabled HTTPS-Redirect but I can’t clone the Repository via my GitHub Desktop-Client.

I imported both root CAs into my Windows trusted ca store, but when I try to clone the Repository in GitHub Desktop with: “https://gitlab.test.local/group1/myfirstrepo.git” I get following error:
schannel: next InitializeSecurityContext failed: Unknown error (0x80092013)

(No problem opening the URL in firefox, cert is trusted here)

I am using a Self-Signed certificate:
-> TestSubRootCA
–> gitlab.test.local

These are the only settings I changed in gitlab.rb:

##GitLab URL
external_url 'https://gitlab.test.local

##GitLab NGINX
nginx[‘redirect_http_to_https’] = true
nginx[‘ssl_certificate’] = "/etc/ssl/certs/gitlab.test.local.pem"
nginx[‘ssl_certificate_key’] = "/etc/ssl/certs/gitlab.test.local.key"
nginx[‘custom_gitlab_server_config’] = 'ssl_password_file “/etc/ssl/auth/pw.txt”;'

Following client-side workaround makes it work, but i wanted to configure it the right way.:
git config --global http.sslBackend "openssl"
git config --global http.sslCAInfo C:/Users/username/ca-bundle.crt
(added both root CAs to it, I dont like this since I wanted to you the Windows Trusted CA Library not the one from OpenSSL)