I have been facing with a strange bug in stunnel 4.55 (and also in tstunnel) in Windows 7 x64 and also in Windows Server 2003 R2 x86. I redirect TCP connections from a win7 client to a win2k3 server. Stunnel works fine when accepting connections and transferring data. But suddenly crashes when the client program (in the client side) closes the connection (maybe the several connections simultaneously). It happens on the server side too. Because of this problem I have to load a little program to relaunch stunnel whenever it is crashed. What I mean by 'crash' is that the windows shows the "stunnel stops working" and giving the choices to restart,stop, or debug the program. If you need more details I will be pleased to help you in this case. Thank you :)
PS: I just have downloaded "stunnel-4.56b1" and I'm using it right now. I see no sign of crashing and instability until this moment :) it seems the bug is eliminated :)