I use Syncthing to sync Joplin between devices and it was working great under Linux and my Android 10, but after a new Android 10 "Lineage" fresh re-install, I started to get this error "joplin could not read directory", I did not touch the Joplin directory and the app has all the permissions, but I noticed that the same error always present with or without the "Storage Permission"!
So, installed "Permission Manager X" and gave Joplin the permission for "LEGACY_STORAGE AppOps, UID mode", and error is gone, the app is syncing just fine now.
I'm sharing this to help resolve this issue and for Joplin team to look into it.