Out of memory

<Try to describe precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.)>
<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 6.3.4

Error message:
Out of memory

Stack trace:
(002A7A1A)
(002A7A1A)
(00225D5B)
(003C8568)
(00CB7EC0)
(00408FE1)
(003C35E8)

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.