Check for Updates behind a Proxy fails

As noted inside the Subject, I can't use the "Check for Updates" feature when using WinSCP here at a side where all internet traffic has to go over a proxy.

I got the following error messages (same behaviour with both methods of starting "Check for Updates"):

Proxy selected in configuration (192.168.1.254 ist the IP of our Proxy):
-> Received response 404 Not Found from 192.168.1.254
No Proxy selected in configuration:
-> Socket error (Timeout)

Additional Information:
Version of WinSCP: Tryed with 5.5.1 and 5.5.3
Version of Microsoft Windows: XP SP3
Transfer protocol: I don't know what protokoll is used by this function, but because of the "404 Not Found ", i guess it's http)

Thanks for your report.
I have sent you an email with a debug version of WinSCP to address you have used to register on this forum.

Sorry, but Your msg containing the debug version was not received here.
I got a message "Topic Reply Notification - Check for Updates behind a Proxy fails" at my e-mail, had points me to this forum message.
So my e-mail should be OK...

I've sent it again.