the main nextcloud.log (what is shown on nextcloud’s UI Logging section) is located:
For instance, using grep to filter results (although log is not so reader-friendly being json one-liners):
runagent -m nextcloud1 podman exec -ti nextcloud-app grep -Ei "(error|warn|fail|miss)" /var/www/html/data/nextcloud.log
or watch the log with tail, then try to access files section and then check what comes up in the log.