<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 5.9.1

Error message:
Copying files to remote side failed.

Invalid pointer operation

Stack trace:
(003E2E58)
(003E2E58)
(008626E1)
(0086448F)
(00758428)
(00757546)
(00783E3D)
(006C50BB)
(006C2896)
(006C0D3C)
(000986B3)
(00012368) kernel32.dll.BaseThreadInitThunk
(00038900) ntdll.dll
(000388D0) ntdll.dll

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.