Thanks for your report.
I have sent you an email with a debug version of WinSCP to address you have used to register on this forum.
Taking a file from WinRAR and dropping in a server side folder in the tree view.
WinSCP 5.7.1
Error message:
List index out of bounds (0)
Stack trace:
[00583685] System::Classes::TList::Get
[00583685] System::Classes::TList::Get
[0063DDA8] Customdriveview::TCustomDriveView::DDProcessDropped
[0065F3EB] Dragdrop::TDragDrop::WndMethod
[005A0708] System::Classes::_18201
[76EA8E6F] USER32.DLL.CallNextHookEx
[76EA90CC] USER32.DLL
[76EAA66A] USER32.DLL
[76EAA6DB] USER32.DLL.DispatchMessageW