<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 6.6.1

Error message:
Invalid access to memory.

Stack trace:
(0043F173) Vcl::Comctrls::TListColumn::DoChange
(005EE534) Jclhookexcept::_16406
(005EE71D) Jclhookexcept::_16409
(00C7798A) ____ExceptionHandler
(000B36B0) ntdll.dll
(000B367F) ntdll.dll
(0006DD2E) ntdll.dll
(0007A5AA) ntdll.dll.KiUserExceptionDispatcher
(00041532) ntdll.dll
(0003801A) gdi32full.dll.GdiRealizationInfo
(0003804E) gdi32full.dll.GdiRealizationInfo
(00093606) ntdll.dll
(00520BAB) Pastools::_16427
(0043F3A0) Vcl::Comctrls::TListColumn::SetWidth
(0004433D) TCustomScpExplorerForm::FormAfterMonitorDpiChanged
(00BEB671) C4927_5
(0003FDD3) USER32.dll
(00026D71) USER32.dll.SendMessageW
(0000028A) win32u.dll.NtUserRedrawWindow
(003CC816) Vcl::Controls::TWinControl::UnlockDrawing
(004D5BED) Vcl::Forms::TCustomForm::DoAfterMonitorDpiChanged
(004D5CAB) Vcl::Forms::TCustomForm::WMDpiChanged
(0003F848) TCustomScpExplorerForm::WMDpiChanged
(00029F7C) USER32.dll
(0003A197) ntdll.dll.RtlDeactivateActivationContextUnsafeFast
(004A071F) Vcl::Themes::TStyleManager::GetIsCustomStyleActive
(004A06F4) Vcl::Themes::TStyleManager::IsCustomStyleAvailable
(000387B1) USER32.dll
(0003F264) TCustomScpExplorerForm::Dispatch
(000275A8) USER32.dll
(000276B2) USER32.dll
(00027642) USER32.dll
(00032AC7) USER32.dll.GetWindowThreadProcessId
(000007CA) win32u.dll.NtUserGetObjectInformation
(003C85BA) Vcl::Controls::TControl::WndProc
(0002970F) UXTHEME.DLL
(003CD765) Vcl::Controls::TWinControl::WndProc
(004D193C) Vcl::Forms::TCustomForm::WndProc
(0002B09C) UXTHEME.DLL
(0003A010) UXTHEME.DLL
(00BEA06A) C4927_0
(00041CDD) ntdll.dll
(00041532) ntdll.dll
(00027A59) USER32.dll
(000006DA) win32u.dll.NtUserSetWindowLong
(000260B4) USER32.dll
(00093606) ntdll.dll
(00042ACA) ntdll.dll.RtlReleaseSRWLockExclusive
(00024369)UXTHEME.DLL
(003CCC70) Vcl::Controls::TWinControl::MainWndProc
(002336B8) System::Classes::_18364
(000387B1) USER32.dll
(000275A8) USER32.dll
(00027293) USER32.dll
(0002704B) USER32.dll
(000AAB77) USER32.dll
(0007A584) ntdll.dll.KiUserCallbackDispatcher
(003CB394) Vcl::Controls::_17058
(003CB513) Vcl::Controls::TWinControl::AlignControls
(004CEA86) Vcl::Forms::TScrollingWinControl::AlignControls
(004D1BFA) Vcl::Forms::TCustomForm::AlignControls
(003D0B9F) Vcl::Controls::TWinControl::GetClientRect
(004D0E07) Vcl::Forms::TCustomForm::GetClientRect
(003CB5FA) Vcl::Controls::TWinControl::AlignControl
(003CE71D) Vcl::Controls::TWinControl::WMSize
(004CF15C) Vcl::Forms::TScrollingWinControl::WMSize
(00166C83) System::TObject::Dispatch
(0003F2DB) TCustomScpExplorerForm::Dispatch
(00022EC2) USER32.dll.GetSystemMetricsForDpi
(003C85BA) Vcl::Controls::TControl::WndProc
(00009F94) GDI32.dll.ExtTextOutW
(00009FA0) GDI32.dll.ExtTextOutW
(000B9ADF) gdi32full.dll
(0002546C) UXTHEME.DLL
(000B9B22) gdi32full.dll.LpkExtTextOut
(0005220E) gdi32full.dll.ExtTextOutWImpl
(000001CA) win32u.dll.NtUserMessageCa

The problem happens rarely. I cannot reproduce it.

@john: Thanks for your report.
I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.

winscp is redrawing the windows every minute, reconnects to sftp server every couple of minutes, interrupt and retakes file transfer, This is behaviour since winscp 6.5. Happens on my HP Z4 workstation with 3 4K monitors. On my laptop everything works flawless. I reinstalled windows but it didn't change the problem.

WinSCP 6.6

Error message:
Invalid access to memory.

Stack trace:
(0043EA33) Vcl::Comctrls::TListColumn::DoChange
(005EDDC0) Jclhookexcept::_16406
(005EDFA9) Jclhookexcept::_16409
(00C754DA) ____ExceptionHandler
(000B3570) ntdll.dll
(000B353F) ntdll.dll
(0006DD0E) ntdll.dll
(0007A7AA) ntdll.dll.KiUserExceptionDispatcher
(00003662)AppHook32_73206A66-D4B5-49D3-B33A-645E29D66BB3.dll
(003A75A5) Vcl::Graphics::TCustomCanvas::TextHeight
(00520419) Pastools::CalculateTextHeight
(00520436) Pastools::_16426
(0052046B) Pastools::_16427
(0043EC60) Vcl::Comctrls::TListColumn::SetWidth
(000441C5) TCustomScpExplorerForm::FormAfterMonitorDpiChanged
(00BE9A3D) C4925_5
(0003F9D3) USER32.dll
(00026941) USER32.dll.SendMessageW
(0000028A) win32u.dll.NtUserRedrawWindow
(003CC0D6) Vcl::Controls::TWinControl::UnlockDrawing
(004D54AD) Vcl::Forms::TCustomForm::DoAfterMonitorDpiChanged
(004D556B) Vcl::Forms::TCustomForm::WMDpiChanged
(0003F6D0) TCustomScpExplorerForm::WMDpiChanged
(00041CDD) ntdll.dll
(000075DA)AppHook32_73206A66-D4B5-49D3-B33A-645E29D66BB3.dll
(0003F174) TCustomScpExplorerForm::Dispatch
(000326B7) USER32.dll.GetWindowThreadProcessId
(000007CA) win32u.dll.NtUserGetObjectInformation
(003C7E7A) Vcl::Controls::TControl::WndProc
(0002965F) UXTHEME.DLL
(000935C6) ntdll.dll
(00052304) ntdll.dll.RtlLeaveCriticalSection
(00051D75) ntdll.dll
(000521B9) ntdll.dll
(003CD025) Vcl::Controls::TWinControl::WndProc
(004D11FC) Vcl::Forms::TCustomForm::WndProc
(0002AFEC) UXTHEME.DLL
(00BE84CA) C4925_0
(00038DA1) UXTHEME.DLL
(00027629) USER32.dll
(000006DA) win32u.dll.NtUserSetWindowLong
(00025C84) USER32.dll
(000935C6) ntdll.dll
(00042ACA) ntdll.dll.RtlReleaseSRWLockExclusive
(000242B9)UXTHEME.DLL
(003CC530) Vcl::Controls::TWinControl::MainWndProc
(00232F78) System::Classes::_18364
(000383B1) USER32.dll
(00027178) USER32.dll
(00026E63) USER32.dll
(00027279) USER32.dll
(00157C23) KERNELBASE.dll.GetProcessMitigationPolicy
(00078304) ntdll.dll.ZwQueryInformationProcess
(001579FA) KERNELBASE.dll.GetProcessMitigationPolicy
(00026C1B) USER32.dll
(000AA437) USER32.dll
(0007A784) ntdll.dll.KiUserCallbackDispatcher
(003CAC54) Vcl::Controls::_17058
(003CADD3) Vcl::Controls::TWinControl::AlignControls
(004CE346) Vcl::Forms::TScrollingWinControl::AlignControls
(004D14BA) Vcl::Forms::TCustomForm::AlignControls
(003D045F) Vcl::Controls::TWinControl::GetClientRect
(004D06C7) Vcl::Forms::TCustomForm::GetClientRect
(003CAEBA) Vcl::Controls::TWinControl::AlignControl
(003CDFDD) Vcl::Controls::TWinControl::WMSize
(004CEA1C) Vcl::Forms::TScrollingWinControl::WMSize
(00166543) System::TObject::Dispatch
(0003F1EB) TCustomScpExplorerForm::Dispatch
(00041CDD) ntdll.dll
(00041532) ntdll.dll
(003C7E7A) Vcl::Controls::TControl::WndProc
(0008FC11) ntdll.dll
(0008FDA2) ntdll.dll
(00091289) ntdll.dll
(00041CDD) ntdll.dll
(0056DDC6) Tb2hook::_16395
(00031E5A) USER32.dll
(003CD025) Vcl:

I know how to reproduce the problem or the problem happens frequently enough. I wish to be contacted by the WinSCP team to help resolving the problem.