<Try to describe precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.)>
Happens not all the time... Windows 10, portable version WinSCP: starting WinSCP I receive the error message (below). If I start WinSCP a second time: no problem.
<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 5.13.4

Error message:
Failed to write ImageList data to stream

Stack trace:
(001A7A9E) Vcl::Listactns::TCustomVirtualListAction::GetItem
(004A8B9A) Jclwin32::EJclWin32Error::EJclWin32Error
(004A7679) Jclsysinfo::_16581
(0086740C) D1576_1
(00867163) D1576_1
(000E2AD8) Vcl::Styles::TCustomStyle::DoGetElementColor
(000024B4) wWinMain
(008BBCC5) __ectbl__ TFTPFileSystem::~TFTPFileSystem
(00062058) ntdll.dll.RtlValidSecurityDescriptor
(00062025) ntdll.dll.RtlValidSecurityDescriptor

The problem happens rarely. I cannot reproduce it.