Invalid access to memory - @$xp$15TEditMaskDialog

<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 6.6

Error message:
Invalid access to memory.

Stack trace:
(0007C7D8) MSFTEDIT.DLL
(005EDDC0)
(005EDFA9)
(00167233)
(000B3570) ntdll.dll
(000B353F) ntdll.dll
(0006DD0E) ntdll.dll
(0007A7AA) ntdll.dll.KiUserExceptionDispatcher
(0007C87A) MSFTEDIT.DLL
(0007C7E4) MSFTEDIT.DLL
(0007C87A) MSFTEDIT.DLL
(00036723) USER32.dll.LoadStringW
(000E5E8E) MSFTEDIT.DLL
(00185D01)
(000E5AFA) MSFTEDIT.DLL
(000E5A87) MSFTEDIT.DLL
(0003F670) USER32.dll
(000276C2) USER32.dll
(00027312) USER32.dll.GetWindowLongW
(000ED0FC) MSFTEDIT.DLL
(001FC7B9) MSFTEDIT.DLL.RichEditWndProc
(0003A0CF) ntdll.dll.RtlActivateActivationContextUnsafeFast
(000383B1) USER32.dll
(00027178) USER32.dll
(00026E63) USER32.dll
(000ED0FC) MSFTEDIT.DLL
(001FC7B9) MSFTEDIT.DLL.RichEditWndProc
(000268E5) USER32.dll.CallWindowProcW
(00027178) USER32.dll
(000268E5) USER32.dll.CallWindowProcW
(003CD12D)
(003ED307)
(00166543)
(00B4E5E4)
(003C7E7A)
(000268E5) USER32.dll.CallWindowProcW
(000383B1) USER32.dll
(003CD12D)
(003ED307)
(00166543)
(00B4E5E4)
(00026E63) USER32.dll
(00027279) USER32.dll
(003C7E7A)
(003CD025)
(003CC530)
(00232F78)
(000383B1) USER32.dll
(00027178) USER32.dll
(00026E63) USER32.dll
(000383B1) USER32.dll
(0003A197) ntdll.dll.RtlDeactivateActivationContextUnsafeFast
(00026C1B) USER32.dll
(00031EE7) USER32.dll
(0007A784) ntdll.dll.KiUserCallbackDispatcher
(000001CA) win32u.dll.NtUserMessageCall
(000284C0) USER32.dll
(0003F9D3) USER32.dll
(00026941) USER32.dll.SendMessageW
(003EC1CB)
(00B4BC67) @$xp$15TEditMaskDialog
(000258D8)
(000935F6) ntdll.dll
(000531E6) ntdll.dll.RtlEnterCriticalSection
(00094CA0)
(000257B0)
(00B4F990)
(0024FC08)
(003B48B9)
(00231FE8)
(00574263)
(00577EB2)
(00582359)
(004D6942)
(004D696A)
(004D69DB)
(004DC432)
(004DC580)
(004DB5E7)
(004DB919)
(0015762D)
(00027F0F) ntdll.dll
(00029B71) USER32.dll
(00029B4C) USER32.dll
(000AE438) ntdll.dll
(004DA451)
(004DACB7)
(00027282) USER32.dll
(00027212) USER32.dll
(00026E63) USER32.dll
(000043AD)
(00BF0A05)
(0004744B) ntdll.dll.RtlInitializeCriticalSectionEx
(00167EE8)
(00164A70)
(00167E2E)
(00168F67)
(00C5CE5D)
(000619FD) ntdll.dll.RtlInitializeCriticalSection
(000935C6) ntdll.dll
(00052304) ntdll.dll.RtlLeaveCriticalSection
(000935C6) ntdll.dll
(00052304) ntdll.dll.RtlLeaveCriticalSection
(00C71D4E)
(002E9390)
(00C6FBA9)
(000A5DB7) @$xp$18TStoredSessionList
(00C711E0)
(00C71529)
(00014D47) KERNEL32.DLL.BaseThreadInitThunk
(0006C819) ntdll.dll
(0006C79C) ntdll.dll

The problem happens rarely. I cannot reproduce it.