My issue is completely unrelated to Excel files. Also it isn't related to files with "." in the middle of them. Perhaps 1 theory is that the original syncing machine (that shared the folder) is left turned off for a day or two while making changes on another machine. When you turn back on the "original" machine it will try to overwrite any files changed on the remote machine even if those files are newer. Just a theory though, haven't confirmed.