In the albums.php code there is a note about how to use curl to update the album

giving this example

curl -b "currenthost=Default;player_backend=mpd" http://localhost/rompr/albums.php?rebuild > /dev/null

but a command based on this does not work giving the following error in the nginx error.log

2020/02/22 01:15:34 [error] 764#764: *6078 FastCGI sent in stderr: "PHP message: PHP Warning: require_once(skins//ui_elements.php): failed to open stream: No such file or directory in /var/www/rompr/backends/sql/backend.php on line 4PHP message: PHP Fatal error: require_once(): Failed opening required 'skins//ui_elements.php' (include_path='.:/usr/share/php') in /var/www/rompr/backends/sql/backend.php on line 4" while reading response header from upstream, client: 127.0.0.1, server: xxxxxxx.localdomain, request: "GET /albums.php?rebuild HTTP/1.1", upstream: "fastcgi://unix:/var/run/php/php7.3-fpm.sock:", host: "xxxxxx"

its failing because it cant load the correct skin as the $skin variable is not set

you need to set the skin in the cookies in the curl command

something like this

curl -b "skin=desktop;currenthost=xxxxxxx;player_backend=mopidy" http://localhost/albums.php?rebuild