@jmichaelmeehan: 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)>
Just doing an upload of all newer or missing files from a very large directory to an sshd server. It uploads several hundred gigabytes before encountering the error below.
WinSCP 5.21.3

Error message:
Invalid access to memory.

Stack trace:
(00C5B3BB)
(00FD2216)
(00FD427E)
(00FD1038)
(00087AFF) ntdll.dll
(00073FA1) ntdll.dll.KiUserExceptionDispatcher
(00C4BEF3)
(00BFF9B2)
(00BFFAA5)
(00D45C22)
(00D44A1C)
(00D93FAF)
(00DD4889)
(00DD4785)
(00DEACF1)
(00010CE9)
(00011908)
(00011B41)
(00011CFF)
(00011E86)
(000481CE)
(0040D5FC)
(001CF6E9)
(003C9BFC)
(0003FBC9) USER32.dll
(00036B85) USER32.dll
(000348F5) USER32.dll
(000346BB) USER32.dll.DispatchMessageW

(00C5B3BB)
(00087AFF) ntdll.dll
(00073FA1) ntdll.dll.KiUserExceptionDispatcher
(00C4BEF3)
(00BFF9B2)
(00BFFAA5)
(00D45C22)
(00D44A1C)
(00D93FAF)
(00DD4889)
(00DD4785)
(00DEACF1)
(00010CE9)
(00011908)
(00011B41)
(00011CFF)
(00011E86)
(000481CE)
(0040D5FC)
(001CF6E9)
(003C9BFC)
(0003FBC9) USER32.dll
(00036B85) USER32.dll
(000348F5) USER32.dll
(000346BB) USER32.dll.DispatchMessageW

Status: unknown_rel_addr

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.