My latest config for gitlab 16.x.x

GITLAB_OMNIBUS_CONFIG: |
        ### Web interface settings
        external_url "https://gitlab.domain.org"
        nginx['listen_port'] = 80
        nginx['listen_https'] = false

        ### Pages
        gitlab_pages['enable'] = true
        pages_external_url "https://pages.domain.org"
        pages_nginx['ssl'] = false
        pages_nginx['listen_https'] = false
        gitlab_pages['listen_proxy'] = '0.0.0.0:8090'
        gitlab_pages['internal_gitlab_server'] = 'http://localhost:8080'

This is a configuration for 1 docker container, which stays behind a reverse proxy(nginx/traefik/whatever). If you terminate https(TLS) on the load balancer it proxies traffic to HTTP ports: 80 - gitlab, 8090 - pages.

I finally figured out what the problem was. Just add ProxyPreserveHost on in the Apache config and everything works nicely. I cannot provide any details about why this did the trick, maybe somebody more experienced with Apache can give some insights.

The full vHost configuration looks like this:

<VirtualHost *:80>

    ServerAlias *.customdomain.com

    <Location />       
        ProxyPreserveHost on
        ProxyPass http://localhost:9081/
        ProxyPassReverse http://localhost:9081/
    </Location>

</VirtualHost>