I may have the same problem as you, is your connection working with other websites? But if you use other sites, they tend not to appear and displays an error instead?
I had the same problem.
The only solve is to disable wingate's proxy and install Squid Proxy.
It is faster, more reliable and more customizable.
You can find it here : [/url]www.squid-cache.org[url]
There is also a win32 port
And...err 500 usualy mean that the server (proxy) did a bubu
Good luck[/url]