This bug has been added to the tracker:
https://winscp.net/tracker/1836

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.

When deleting some thousands of locally stored files in WinSCP 5.15.3 on my I7 16GB RAM - Win7 64bit
after some hours of waiting I get:
Error message:
Invalid access to memory.
Stack trace:
(00812F04)
(0092770A)
(00929772)
(0092652C)
(0006246E) ntdll.dll
(0000F12E) ntdll.dll.KiUserExceptionDispatcher
(0081E4FB)
(0081F496)
(000104C1)
(0001057B)
(0001070C)
(00010793)
(00042346)
(003E7824)

(00927D75)
(0092934F)
(00015DEA)
(000104C1)
(0001057B)
(0001070C)
(00010793)
(00042346)
(003E7824)

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.