On following the link to the Help page [https://winscp.net/eng/docs/message_invalid_access_to_memory], the information therein states: "This error message is not useful for you as an end-user. It generally means that there is a bug in the software. Please report the bug."
The issue occurred repeatedly using "F5 Copy". However when the file was transferred via Drag&Drop, there was no issue.
OS: Windows 7 Enterprise 32-bit
WinSCP: 5.1.1 (Build 2735)
Transfer protocol: SFTP