@rubbia: As above: Please describe step by step how to reproduce your problem, including the external editor configuration in WinSCP. Some screenshots would help. Better if you start a new thread for this (post a link here, if you do).
Before posting, please read how to report bug or request support effectively.
Bug reports without an attached log file are usually useless.
C:\Temp\WinSCP, and it works now. Preferences - > Storage - > Temporary directory, set path in "Use this directory".