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.
Great! Glad you've managed to track down the issue! Yes, "Can't open the destination folder" is not a very user-friendly error - it would be much clearer if this said "Please select a folder to sync" or similar!
Hopefully the team will re-word this error, but in the meantime, as it appears to be catching a number of users out, I've now added an Unofficial FAQ
entry to clarify the meaning of this error!