Setting up Nextcloud with other Applications and SSL/https

So it does now work. I had to combine the two .conf files to one and make a symbolic link from the /opt/… to /var/www/openproject. I used

sudo ln -s /opt/openproject/public /var/www/opp

to create the Link.
Now the only issue left is that the Document root is still accessible… i tried to use a .htaccess file to block this with a “deny All”, but this doesn’t change anything.
For futur references here is my resulting .conf:

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

<VirtualHost *:80>
  ServerName my.dyndns.com
  RewriteEngine On
  RewriteRule ^/?(.*) https://%{SERVER_NAME}:443/$1 [R,L]
  Alias /nextcloud "/var/www/nextcloud/"
  Alias /opp "/var/www/opp/"
  <Directory "/var/www/nextcloud/">
    Options +FollowSymlinks
    AllowOverride All
 
    <IfModule mod_dav.c>
      Dav off
    </IfModule>
 
    Require all granted
 
    SetEnv HOME /var/www/nextcloud
    SetEnv HTTP_HOME /var/www/nextcloud
  </Directory>

  <Directory "/var/www/opp/">
    Options +FollowSymlinks
    AllowOverride All
 
    <IfModule mod_dav.c>
      Dav off
    </IfModule>
 
    Require all granted
    SetEnv HOME /var/www/opp
    SetEnv HTTP_HOME /var/www/opp
  </Directory>
ErrorLog ${APACHE_LOG_DIR}/server13_error_log
CustomLog ${APACHE_LOG_DIR}/server13_access_log common
RewriteEngine on
RewriteCond %{SERVER_NAME} =my.dyndns.com
RewriteRule ^ https://%{SERVER_NAME}%{REQUEST_URI} [END,NE,R=permanent]
</VirtualHost>

<VirtualHost *:443>
  ServerName my.dyndns.com
  DocumentRoot /var/www/
  #DocumentRoot /opt/openproject/public #original root
  Alias /nextcloud "/var/www/nextcloud/"
  Alias /opp "/var/www/opp/"
  <Directory "/var/www/nextcloud/">
    Options +FollowSymlinks
    AllowOverride All
 
    <IfModule mod_dav.c>
      Dav off
    </IfModule>
 
    Require all granted
 
    SetEnv HOME /var/www/nextcloud
    SetEnv HTTP_HOME /var/www/nextcloud
  </Directory>

  <Directory "/var/www/opp/">
    Options +FollowSymlinks
    AllowOverride All
 
    <IfModule mod_dav.c>
      Dav off
    </IfModule>
 
    Require all granted
    SetEnv HOME /var/www/opp
    SetEnv HTTP_HOME /var/www/opp
  </Directory>

  ProxyRequests off

  Include /etc/openproject/addons/apache2/includes/vhost/*.conf
  SSLCertificateFile /etc/letsencrypt/live/my.dyndns.com/fullchain.pem
  SSLCertificateKeyFile /etc/letsencrypt/live/my.dyndns.com/privkey.pem
  Include /etc/letsencrypt/options-ssl-apache.conf
  # Can't use Location block since it would overshadow all the other proxypass directives on CentOS
  ProxyPass /opp/ http://127.0.0.1:6000/opp/ retry=0
  ProxyPassReverse /opp/ http://127.0.0.1:6000/opp/
</VirtualHost>

Thank you all for helping me!