Created Unassigned: Access denied for local file during upload [17]

I'm trying to upload some files from a local folder on my machine to a library in my sharepoint site.

The problem is that after working flawlessly for about 40 files, now SPFileZilla seems to be denied access to a couple files under that same local folder during upload. The error message is the following:

System.UnauthorizedAccessException: Access to the path 'C:\docs\Utilities.pdf' is denied.
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, String msgPath, Boolean bFromProxy)
at System.IO.FileStream..ctor(String path, FileMode mode)

The thing is, there's about 8 files in the c:\docs folder, and only one specific file can be uploaded. The rest all gives me this access denied error.

As far as I know, I'm running this machine as administrator. I have full admin permission for the docs folder. I've also tried to run SPFileZilla as administrator, but it still gives me the same error with those files. I've tried to remove those files from the then put some other files into the docs folder, and they can be uploaded through SPFileZilla without error.

So what might be the cause for this?