I have multiple family trees and was manually syncing them today. The first tree, Torgerson, synced fine. But when syncing the 2nd tree, Lystad, I got the Sync Error. When I looked in the Sync area within FTM, it said Torgerson for "Online Tree". Clearly not right. So I went into the Family Tree Maker folder (using Windows Explorer) and renamed the Torgerson file so that FTM wouldn't recognize it. (You could also move the file into another folder.) After that I was able to sync my Lystad tree successfully. I then, of course, renamed my Torgerson file back.
I'm using FTM 2012.