@nino1337 I was able to reproduce your setup with the help of this thread (working example for Uberspace in a secondary docs root, but not in a subfolder):
This works, if your cockpit instance is in a subfolder /cms
:
# connect domain
uberspace web domain add test.domain.tld
# set A and AAAA records at your domain provider...
# create docs root
cd /var/www/virtual/$USER
mkdir test.domain.tld
cd test.domain.tld
git clone https://github.com/agentejo/cockpit.git cms
cd cms
nano .htaccess
# set RewriteBase /cms
# Be aware, that .htaccess might be overridden with the next cockpit update
nano defines.php
# set constants (see below)
defines.php:
<?php
define('COCKPIT_BASE_URL', '/cms');
define('COCKPIT_BASE_ROUTE', '/cms');
define('COCKPIT_DOCS_ROOT', dirname(__DIR__));
define('COCKPIT_SITE_DIR', dirname(dirname(__DIR__)));