Nextcloud 503 error

I followed the instructions in to add an external drive and to install Nextcloud (including rebooting the router). It seemed to all go well, but now I get " 503 - Service Not Available" error when I attempt to access it via https://[router ip]/nextcloud/

If I look at https://[router ip]/foris/config/main/diagnostics/ I don’t see any errors, but neither do I see lighthttpd, mariadb, nor nextcloud.

How can I diagnose/fix this problem?