External exception EEFFACE

<Try to describe precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.)>
I was copying a large number of files from a hard drive to an external server. When I checked on the transfer I got this error
<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 6.1

Error message:
External exception EEFFACE

Stack trace:
(0013C8A2)KERNELBASE.dll
(00087AEF)ntdll.dll
(00073FB1)ntdll.dll
(00543898)
(0100545E)
(0100684F)
(00DEE8BB)
(00DE560E)
(00E18277)
(00E16BA0)
(00E1712D)
(00E17FF1)
(00E16BA0)
(00E1712D)
(00E17FF1)
(00E16BA0)
(00E1712D)
(00E17FF1)
(00E16BA0)
(00E1712D)
(00E17FF1)
(00E16BA0)
(00E169CD)
(00DE2182)
(00E1642C)
(000113AF)
(00011FD4)
(000279E5)
(00027B7D)
(00C9362A)
(0047B29C)
(004AFD57)
(00214D34)
(00040379)USER32.dll
(00037345)USER32.dll
(000350B5)USER32.dll
(00034E7B)USER32.dll

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.