Thanks for your report.

Can you send me an email, so I can send you back a debug version of WinSCP to track the problem? Please include link back to this topic in your email. Also note in this topic that you have sent the email. Thanks.

OS:Windows 8.1 x64
WinSCP version: 5.7.1 build 5235

Stack trace:
[00F60F2A] Tbxoffice2003theme::PaintIrregularGradient
[77B8023E] ntdll.dll.ZwWow64CallFunction64
[77B809BA] ntdll.dll.KiUserExceptionDispatcher
[00F617BE] Tbxoffice2003theme::TTBXOffice2003Theme::PaintBackgnd
[00F51807] Tbx::TTBXToolbar::WMEraseBkgnd
[00D37CA1] Vcl::Controls::TControl::WndProc
[00D3C7E5] Vcl::Controls::TWinControl::WndProc
[00D3BE2C] Vcl::Controls::TWinControl::MainWndProc
[00E60708] System::Classes::_18201
[772B8E6F] USER32.DLL.CallNextHookEx
[772BAEF8] USER32.DLL
[772B9327] USER32.DLL
[772B9527] USER32.DLL
[77B80994] ntdll.dll.KiUserCallbackDispatcher
[00F2E7DB] Tb2dock::TTBCustomDockableWindow::SetBounds
[00F2C2DA] Tb2dock::TTBDock::ArrangeToolbars
[00F2B4C2] Tb2dock::TTBDock::AlignControls
[00D3AAA2] Vcl::Controls::TWinControl::Realign
[00D3A977] Vcl::Controls::TWinControl::AlignControls
[00D3AA46] Vcl::Controls::TWinControl::AlignControl
[00D3D783] Vcl::Controls::TWinControl::WMSize
[00D3C7E5] Vcl::Controls::TWinControl::WndProc
[00D3BE2C] Vcl::Controls::TWinControl::MainWndProc
[00E60708] System::Classes::_18201
[772B8E6F] USER32.DLL.CallNextHookEx
[772B90CC] USER32.DLL
[772B9327] USER32.DLL
[772B9527] USER32.DLL
[77B80994] ntdll.dll.KiUserCallbackDispatcher
[772BD59A] USER32.DLL
[772BB99A] USER32.DLL
(00001E19) UXTHEME.DLL.[73682E19]
(00001E53) UXTHEME.DLL.[73682E53]
[772BB933] USER32.DLL
[772B8E6F] USER32.DLL.CallNextHookEx
[772B90CC] USER32.DLL
[772BDDD0] USER32.DLL.CallWindowProcW
[00D3C8F6] Vcl::Controls::TWinControl::DefaultHandler