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.

<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 5.7.6

Error message:
External exception EEFFACE

Stack trace:
(000D1ED2) KERNELBASE.dll.RaiseException
(000A267F) ntdll.dll
(0009018A) ntdll.dll.KiUserExceptionDispatcher
(0049641C) Jclhookexcept::_16408
(007E4F0E) C4799_3
(007E62FF) _ReThrowException
(00659E58) TFTPFileSystem::DoWaitForReply
(00659ED7) TFTPFileSystem::WaitForReply
(00659F30) TFTPFileSystem::WaitForCommandReply
(006546D8) TFTPFileSystem::Source
(00653DA2) TFTPFileSystem::SourceRobust
(00653B98) TFTPFileSystem::CopyToRemote
(00723225) TTerminal::CopyToRemote
(0000C302) TCustomScpExplorerForm::ExecuteFileOperation
(0001D2A9) TCustomScpExplorerForm::RemoteFileControlDragDropFileOperation
(0001D432) TCustomScpExplorerForm::RemoteFileControlDDFileOperation
(0061488A) TUnixDirView::PerformItemDragDropOperation
(003D969E) Customdirview::TCustomDirView::DDProcessDropped
(003FEF8B) Dragdrop::TDragDrop::WndMethod
(003402A8) System::Classes::_18201
(00034B81) USER32.dll
(00018D15) USER32.dll
(0001885B) USER32.dll
(000186AB) USER32.dll.DispatchMessageW

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.