<Try to describe precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.)>

Just selected a bunch of folders in my Windows 10 explorer window and dragged it inside winscp remote folder (directly inside the right side of the "Explorer Layout").
A second instance of Winscp was active, with enabled folder snchronisation.

After restarting WinSCP everything is working!

<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 5.8.1

Error message:
Invalid access to memory.

Stack trace:
(0062723D)
(008300A2)
(0083210A)
(0082F084)
(0008A61F) ntdll.dll
(00077E7A) ntdll.dll.KiUserExceptionDispatcher
(00624712)
(00624C17)
(00628990)
(00628A61)
(005F4C71)
(005FAE2F)
(00607805)
(006C9AFA)
(0074376E)
(00696CC8)
(006951A4)
(00092057)
(000128F2) KERNEL32.DLL.BaseThreadInitThunk
(00064DE1) ntdll.dll
(00064DA9) ntdll.dll

(0083070D)
(00831CE7)
(006C9CB7)
(0074376E)
(00696CC8)
(006951A4)
(00092057)
(000128F2) KERNEL32.DLL.BaseThreadInitThunk
(00064DE1) ntdll.dll
(00064DA9) ntdll.dll

(0083070D)
(00831CE7)
(00743898)
(00696CC8)
(006951A4)
(00092057)
(000128F2) KERNEL32.DLL.BaseThreadInitThunk
(00064DE1) ntdll.dll
(00064DA9) ntdll.dll

(0083070D)
(00831CE7)
(00743C9A)
(00696CC8)
(006951A4)
(00092057)
(000128F2) KERNEL32.DLL.BaseThreadInitThunk
(00064DE1) ntdll.dll
(00064DA9) ntdll.dll

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.