Loving me some Syncthing!
Posting this for those who find themselves in this position.
I recently had the need to move the default Sync to another drive because I had run out of space on my Odroid 8 gig SD card. I moved all the files from ~/Sync to an external hard-drive with the same name and made a symbolic link to the new folder.
I was immediately presented with the “stopping folder path exists but folder marker missing” error message.
Everything quit syncing.
The fix was to create a “.stfolder” in the new folder on the external drive. Then restart all instances of syncthing on the devices using it.
Hope this helps someone.