<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 5.15.5

Error message:
Stream write error

Stack trace:
(0038F1FF)
(0038F183)
(0038A1CC)
(0073CBB8)
(007C7D1F)
(007C7D54)
(0088240B)
(008825EB)
(002791E1)
(001E1370)
(00279347)
(002791E1)
(00278828)
(003A3E44)
(00045249) USER32.dll
(000364D7) USER32.dll
(00036075) USER32.dll
(0004260D) USER32.dll
(000718EB) ntdll.dll.KiUserCallbackDispatcher
(0003594E) USER32.dll.SendMessageW
(00044AB9) COMCTL32.dll
(00056F9A) COMCTL32.dll.SetWindowSubclass
(00045249) USER32.dll
(000364D7) USER32.dll
(00035A2A) USER32.dll.CallWindowProcW
(002792F2)
(002791E1)
(001E1370)
(003A3E44)
(00045249) USER32.dll
(000364D7) USER32.dll
(00035616) USER32.dll
(000353EB) USER32.dll.DispatchMessageW

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.