WireGuard SFTP issues + Invalid memory access - _count234

This only happens when I am connected through WireGuard VPN to my (FreeBSD) ssh daemon.
Normal SFTP and SSH operations work fine. But once willing to upload multiple (10) small text files, the connection is not working anymore. I get «unable to connect» errors, or WinSCP whining about the wrong key values of some sort.

I thought this was an MTU issue, and modified my settings to the maximum MTU in each direction. MTU = 1392 on my VPN client, MTU = 1472 on my VPN server interface (wgsrv1).

After that change, the problem was worse. WinSCP is now giving me an invalid memory access.

I also tried disabling the automatic buffer adaptation.

I am not sure if this is a WinSCP, WireGuard or Unifi UDP SE (my VPN router) issue.

WinSCP 5.21.7

Error message:
Ongeldige toegang tot geheugen.

Stack trace:
(00C6C801) C2842_0
(00FDF7C2) C5412_0
(00FE182A) ____ExceptionHandler
(00FDE5E4) __ExceptionHandler
(00087B7F) ntdll.dll
(00074001) ntdll.dll.KiUserExceptionDispatcher
(00C68CDC) _count234
(00C691E3) _add234
(00C6DECB) _sk_addr_dup
(00C6DFCD) _putty_sk_new
(00C42586) _new_connection
(00C56FAF) _ssh_sw_abort_deferred
(00C5728B) _ssh_is_bare
(00D49A5B) TSecureShell::Open
(00DD7641) TTerminal::Open
(00D02F44) TTerminalItem::ProcessEvent
(00D012C0) TSignalThread::Execute
(000C282F) Userinterface::C20_0
(0001F0F7) KERNEL32.DLL.BaseThreadInitThunk
(00066BBC) ntdll.dll.RtlGetAppContainerNamedObjectPath
(00066B89) ntdll.dll.RtlGetAppContainerNamedObjectPath

(00FDFE2D) C5412_0
(00FE1407) _ReThrowException
(00D49C3A) TSecureShell::Open
(00DD7641) TTerminal::Open
(00D02F44) TTerminalItem::ProcessEvent
(00D012C0) TSignalThread::Execute
(000C282F) Userinterface::C20_0
(0001F0F7) KERNEL32.DLL.BaseThreadInitThunk
(00066BBC) ntdll.dll.RtlGetAppContainerNamedObjectPath
(00066B89) ntdll.dll.RtlGetAppContainerNamedObjectPath

(00FDFE2D) C5412_0
(00FE1407) _ReThrowException
(00DD77E9) TTerminal::Open
(00D02F44) TTerminalItem::ProcessEvent
(00D012C0) TSignalThread::Execute
(000C282F) Userinterface::C20_0
(0001F0F7) KERNEL32.DLL.BaseThreadInitThunk
(00066BBC) ntdll.dll.RtlGetAppContainerNamedObjectPath
(00066B89) ntdll.dll.RtlGetAppContainerNamedObjectPath

(00FDFE2D) C5412_0
(00FE1407) _ReThrowException
(00DD7E55) TTerminal::Open
(00D02F44) TTerminalItem::ProcessEvent
(00D012C0) TSignalThread::Execute
(000C282F) Userinterface::C20_0
(0001F0F7) KERNEL32.DLL.BaseThreadInitThunk
(00066BBC) ntdll.dll.RtlGetAppContainerNamedObjectPath
(00066B89) ntdll.dll.RtlGetAppContainerNamedObjectPath

The issue does not seem to be WireGuard related. From within the same LAN I have the same issues, but to a less obvious extent.

Restarting the SSH daemon resolved this issue.

Thanks for sharing your findings.