No, the Nextcloud and WebDAV settings were blank.
But, in WebDAV, I enabled "Ignore TLS certificate errors", and then changed the target back to Dropbox. And that worked - it's syncing properly now.
I'd suggest moving that "Ignore TLS..." option outside of the targets, since it's currently only visible under WebDAV and Nextcloud.
Anyway, that was a huge help - thank you! 
