As posted above I had this "Can't open the destination folder" error" problem. After struggling using three different computers, and trying to sync different folders, I found my problem. When trying to sync with the second and third computers I was leaving the "folder to sync" field empty. I don't know why, but I thought that this field should only be filled in on the first computer to direct what files should be shared. Its only later I thought "but where does the synced files go on the second computer?" that I realised the other computers should also have a destination folder to sync. Once I set everything up properly, and waited a few minutes, the folders synced and updated.