sure lets give it a try.
The download button is only displayed when the call is_path_accessible( Path::get_path() )
returns true, see functions/core.php
@ line 198 in the plugin directory. Try adding the following lines at the top of that function:
file_put_contents( Path::get_path() . '/debug.txt', 'check if ' . $dir . ' contains ' . Path::get_home_path() );
return true;
The Download button should be there now, does it work? Check the debug.txt created in your backup directory and let us know what it says, on a working machine it looks something like this:
check if /var/www/html/wp-content/backupwordpress-17f54d7318-backups contains /var/www/html