OK now, I got the site/page to work with Opera 7.23, IE 6.0 and Mozilla Firebird 0.7. Here's what I did:

1) Tried to load the page with Opera - a proxy error about server couldn't be found
2) Tried to load the page with Opera, Proxomitron disabled - same result
3) Tried to load the page with Opera, Proxo and proxy disabled - same result
4) Emptied Opera's cache and made it check documents always - it worked!

It was basically the same routine with all the browsers. If it first didn't work, then change connection type to 'direct connection to internet' (ie. turn proxy off) and empty your cache.

So it doesn't seem to be an OS or browser related problem. Probably some proxies just don't know how to route to that particular address.