Invalid Access to memory

Stack trace:
(0060AF75) C2280_0
(007E39EA) C4799_0
(007E5A52) ____ExceptionHandler
(007E29CC) __ExceptionHandler
(0007C61F) ntdll.dll
(00069E6A) ntdll.dll.KiUserExceptionDispatcher
(006088FE) _count234
(00608E03) _add234
(0060C5FC) _sk_register
(0060C6CD) _putty_sk_new
(005E22D9) _new_connection
(005E751E) C2232_0
(005F2E34) _sshfwd_unthrottle
(006A2806) TSecureShell::Open
(0070BA56) TTerminal::Open
(00670C4C) TTerminalItem::ProcessEvent
(0066F128) TSignalThread::Execute
(00082687) Userinterface::C21_0
(00012742) KERNEL32.DLL.BaseThreadInitThunk
(00059062) ntdll.dll
(0005902A) ntdll.dll

(007E4055) C4799_0
(007E562F) _ReThrowException
(006A29C3) TSecureShell::Open
(0070BA56) TTerminal::Open
(00670C4C) TTerminalItem::ProcessEvent
(0066F128) TSignalThread::Execute
(00082687) Userinterface::C21_0
(00012742) KERNEL32.DLL.BaseThreadInitThunk
(00059062) ntdll.dll
(0005902A) ntdll.dll

(007E4055) C4799_0
(007E562F) _ReThrowException
(0070BB80) TTerminal::Open
(00670C4C) TTerminalItem::ProcessEvent
(0066F128) TSignalThread::Execute
(00082687) Userinterface::C21_0
(00012742) KERNEL32.DLL.BaseThreadInitThunk
(00059062) ntdll.dll
(0005902A) ntdll.dll

(007E4055) C4799_0
(007E562F) _ReThrowException
(0070BF82) TTerminal::Open
(00670C4C) TTerminalItem::ProcessEvent
(0066F128) TSignalThread::Execute
(00082687) Userinterface::C21_0
(00012742) KERNEL32.DLL.BaseThreadInitThunk
(00059062) ntdll.dll
(0005902A) ntdll.dll

The problem happens rarely. I cannot reproduce it.