Clone via HTTPS returns "The requested URL returned error: 500"

I installed Phabricator via a docker container and reverse proxy to it via nginx. SSH cloning works without a flaw but https failes with the following message:

fatal: unable to access 'https://phabricator.xxx.yyy/diffusion/2/testrepo.git/': The requested URL returned error: 500

these are the configs from my nginx:
Reverse Proxy:

server {
listen 443 ssl http2;
listen [::]:443 ssl http2;

server_name phabricator.xxx.yyy;

include /config/nginx/proxy-confs/*.subfolder.conf;

include /config/nginx/ssl_xxx.conf;

client_max_body_size 0;
location / {
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection 'upgrade';
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_cache_bypass $http_upgrade;

    proxy_pass http://phabricator; # Note: Trailing slash
}
}

Phabricator:

server {
  server_name phabricator.xxx.yyy;
  root /var/www/phabric/phabricator/webroot/;

  location / {
    index index.php;
    rewrite ^/(.*)$ /index.php?__path__=/$1 last;
  }

  location /index.php {
    fastcgi_pass   unix:/run/php/php7.4-fpm.sock;
    fastcgi_index   index.php;

    #required if PHP was built with --enable-force-cgi-redirect
    fastcgi_param  REDIRECT_STATUS    200;

    #variables to make the $_SERVER populate in PHP
    fastcgi_param  SCRIPT_FILENAME    $document_root$fastcgi_script_name;
    fastcgi_param  QUERY_STRING       $query_string;
    fastcgi_param  REQUEST_METHOD     $request_method;
    fastcgi_param  CONTENT_TYPE       $content_type;
    fastcgi_param  CONTENT_LENGTH     $content_length;

    fastcgi_param  SCRIPT_NAME        $fastcgi_script_name;

    fastcgi_param  GATEWAY_INTERFACE  CGI/1.1;
    fastcgi_param  SERVER_SOFTWARE    nginx/$nginx_version;

    fastcgi_param  REMOTE_ADDR        $remote_addr;
  }
}

You should have a matching error logged in the webserver’s log or in fastcgi’s log (the one where Phabricator is running).