SSL Issue on integration of Nextcloud with OpenProject

Question

I hope some had a similar problem and could solve it. I try to bind Nextcloud and OpenProject together but cant get it to work.

the following picture shows the setup:

I configured (or even tried to) both services as mentioned in their documentation. I use Node-Package-Manager as a proxy service

I use a wildcard certificate (no self-signed certificate) and at openproject i have set the following custom settings:

proxy_set_header X-Forwarded-Host $host:$server_port;
proxy_set_header X-Forwarded-Proto https;

The Apache serve shipping with openproject was configured by the installation steps

Include /etc/openproject/addons/apache2/includes/server/*.conf
IncludeOptional /etc/openproject/addons/apache2/custom/server/*.conf

<VirtualHost *:80>
  ServerName openproject.example.net
  DocumentRoot /opt/openproject/public

  ProxyRequests off

  Include /etc/openproject/addons/apache2/includes/vhost/*.conf
  IncludeOptional /etc/openproject/addons/apache2/custom/vhost/*.conf

  # Can't use Location block since it would overshadow all the other proxypass directives on CentOS
  ProxyPass / http://127.0.0.1:6000/ retry=0
  ProxyPassReverse / http://127.0.0.1:6000/
</VirtualHost>

Based on this description external SSL termination

I created the OpenProject configuration. At the nextcloud i have set the following custom settings:

client_body_buffer_size 512k;
proxy_read_timeout 86400s;
client_max_body_size 0;
proxy_set_header Host $host;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
add_header Front-End-Https on;

Based on teh discription here Nextcloud configuration - Reverse proxy I edited the config.php like so:

  'overwrite.cli.url' => 'https://nextcloud.example.net',
  'trusted_proxies'   => ['192.168.1.44'],
  'overwritehost'     => 'nextcloud.example.net',
  'overwriteprotocol' => 'https',
  'overwritewebroot'  => '/',
  'overwritecondaddr' => '^192\.168\.1\.44$',

The tests on this site integration - troubleshooting

curl --location --request GET 'https://nextcloud.example.net/index.php/apps/integration_openproject/check-config' --header 'Authorization: foo'

resulted in the following error

curl: (60) SSL certificate problem: unable to get local issuer certificate
More details here: https://curl.se/docs/sslcerts.html

curl failed to verify the legitimacy of the server and therefore could not
establish a secure connection to it. To learn more about this situation and
how to fix it, please visit the web page mentioned above.

As mentioned before I tried all the configurations but nothing helped to establish a connection. I don't know what I'am missing and what I could try to get this working together...