Xopero application uses self-signed certificate, but it is also possible to use your own.
Changing the certificate
Using SFTP client connect with QNAP device.
Default admin/admin, port 22
Go to /share/CACHEDEV1_DATA/.qpkg/XoperoBackup/Nginx/nginx/conf and find 2 files: server.crt and server.key. Replace them with new certificate's files.
After the files are changed, restart Xopero application: