Invalid access to memory

Simply copying files from server to desktop via SFTP-3

WinSCP 6.1.1

Error message:
Invalid access to memory.

Stack trace:
(00FF1FBF)
(0100350E)
(01005576)
(01002330)
(00087A5F) [77

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.