Error code 1 when syncing with Google Drive (Windows)

[kkew]

I have been synchronizing a local folder with a folder in the virtual hard disk created by Google Drive Windows 10 client. However, FreeFileSync keeps complaining about "Error code 1" and "[CopyFileEx]", and then terminate synchronization. In the log, the files resulting in such problem usually have extension name ".gdoc" or ".ffs_tmp", and they cannot be found on either side of the synchronization pair. What's going on? Thanks in advance.