<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 5.11.3

Error message:
Stream write error

Stack trace:
(0036712F)
(003670B3)
(003620FC)
(006D4970)
(0068DD45)
(0068DC37)
(000024FD)
(0089B381)
(00063A75) ntdll.dll.RtlGetAppContainerNamedObjectPath
(00063A42) ntdll.dll.RtlGetAppContainerNamedObjectPath

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.