I finally managed it to add the external storage into Nextcloud. All working! Even through the app.
The solution was to add the Apache user www-data to the group vboxsf (which is containing the users that have full access to the shared folders), then granting full access to the folder /var/www/html/data/ and or the shared sf_folders.
Use theses commands in Ubuntu:
sudo chown www-data /media/sf_Programme
sudo chown www-data /media/sf_Bilder
sudo adduser www-data vboxsf
sudo bash -c "chmod a=rwx /var/www/html/data/"
After that a quick reboot, then everything worked out just fine!
