I'm getting this same issue and the exception is below.
/www.mywebsite.com/ftp/FileToResume.mpg: invalid REST argument (554).
at Rebex.Net.Ftp.CgtFNEZ(String , AoLBks , String , String , Int64 , FtpTransferState )
at Rebex.Net.Ftp.bjMtzmZ(String , Stream , Int64 , String )
at Rebex.Net.Ftp.GetFile(String remotePath, Stream outputStream, Int64 remoteOffset)