You can work around that by replacing ftp:// prefix with command line parameter /RawSettings FSProtocol=5
WinSCP.exe user:password@10.10.10.10 /RawSettings FSProtocol=5
/passive /log="d:\connectionlogs\10.10.10.10_2012_10_02_winscp.log
thanks for the workaround with
WinSCP.exe user:password@10.55.35.46 /RawSettings FSProtocol=5 PortNumber=21 /passive /log="logfile"
it works.