GitLab self-hosted behind NGINX Reverse-Proxy doesn't work with Azure AD Integration

Hello,

i want to integrate my self-hosted GitLab CE Instance with Microsoft Azure Authentication. I strictly followed this: Microsoft Azure OAuth2 OmniAuth Provider | GitLab Guide Microsoft Azure OAuth2 OmniAuth Provider v2

GitLab.com and Google Integrations are working just fine.

Thats the Error im getting:
AADSTS50011: The reply URL specified in the request does not match the reply URLs configured for the application

Reverse Proxy Configuration

server {
    server_name gitlab.example.com www.gitlab.example.com;
    location / {
        proxy_pass http://localhost:8929;
        proxy_set_header Host $host;
    }

    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/gitlab.example.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/gitlab.example.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
}

GitLab OmniAuth Configuration


### OmniAuth Settings
gitlab_rails['omniauth_enabled'] = true
gitlab_rails['omniauth_allow_single_sign_on'] = ['saml']
gitlab_rails['omniauth_providers'] = [
   {
     "name" => "azure_activedirectory_v2",
     "args" => { 
         "client_id" => "...",
         "client_secret" => "...",   
         "tenant_id" => "..."
        },
   },
   {
     "name" => "gitlab",
     "app_id" => "...",
     "app_secret" => "...",
     "args" => { "scope" => "api" }
   },
   {
     "name" => "google_oauth2",
     "app_id" => "...",
     "app_secret" => "...",
     "args" => { "access_type" => "offline", "approval_prompt" => '' } 
   } 
]

I thought that the reverse-proxy might be breaking it.
Anyone else experienced something simliar?