If you are able to Flame cache on non Lucid Link file paths, but unable to Flame cache from Lucid Link, that is expected behavior, as by default the Lucid Link config file only lets your user permission to access the Lucid Link file space, not the Backburner user.
This was briefly touched on by CNoellert, hither.
See in the 7th line of his ~/.lucid/app.json config file?
“fuseAllowOther”: true
You need to make your app.json file have that line, and boom, you can Flame cache media from Lucid Link.