<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 6.3.4

Error message:
External exception E06D7363

Stack trace:
(001488B2) KERNELBASE.dll.RaiseException
(000987BF) ntdll.dll
(00077E51) ntdll.dll.KiUserExceptionDispatcher
(00006E90) VCRUNTIME140.dll._CxxThrowException
(000C8238) clpbm.dll
(00002FBE)clpbm.dll
(00002CDA)clpbm.dll
(00045458) clpbm.dll
(0000A4F0)clpbm.dll
(00009DCC)clpbm.dll
(0000E272)clpbm.dll
(00031402) TCustomScpExplorerForm::ClipboardClear
(000611B8) TScpCommanderForm::CopyFilesToClipboard
(0004F416) TNonVisualDataModule::ExplorerActionsExecute
(002757CC) System::Actions::TContainedActionList::ExecuteAction

The problem happens rarely. I cannot reproduce it.