I've noticed this behaviour on more than one PC running WinSCP 4.1.5 (Build 410). All PCs are running Windows XP Pro with SP3 and the latest updates from M$.
Here's the scenario:
start a download (1 file or many; I always use Background) and let it run to completion, and then let WinSCP get disconnected due to inactivity.
WinSCP displays a dialog box notifying user of disconnection. Buttons in the box are OK, RECONNECT and... I forget. RECONNECT will re-establish the connection without error. Acknowledging the box by clicking OK causes the "WinSCP Login" box to pop up, with Stored Sessions in view.
Clicking the Login button causes WinSCP to hang, eventually displaying the Not Responding message (in the title bar, I think). IIRC, the Login dialog box goes away, and it looks like I can interact with the server, but attempting to do so gets no reaction. Clicking the X in the upper right corner to close the app causes the Windows "End Now" dialog box to appear, which does kill the app. No problem relaunching WinSCP and logging into the same server.
This is all done using the Commander interface. I didn't have logging turned on, but I am attempting to use it now. If successful I will update this post.