I have made a full copy of a Windows FAT file system to a NetApp Cifs share and that was done successfully. After the initial copy i wanted to use the sync command to keep de copy the same as the source.
As command i used this (source - and destination server and share are correct but removed in this line. But i know these are good cause i used the same for the initial copy by replaceing copy command to sync command)
The first time your destination was empty. The second it wasn't, and you have FAT on one side and NTFS-like behavior on the other. Plus your user may not even be admin or have sufficient permissions, etc.
You can try to sync the same source data to another place twice to see if there's something specific to resync.
You can also run "xcp help sync" to see options at your disposal and try some of them.
You can use "xcp show \\source\share" to see properties of existing shares at the source and destination.
You can try to use general SMB commands to see if your account permissions are enough to touch/create/delete files at the source and destination.
You can try to create another such pair of shares and verify the process works correctly the first time.