What causes the 407 error on page?

[shabbysquire]

On the template in ZP, I sometimes get server code: 407 (proxy auth) where the page is supposed to be displayed. Like this:

Just thought I'd ask as I've never come across this before (ver. 5.5.0), and all my proxies are quite stable.

 

[rostonix]

It's proxy problem http://www.checkupdown.com/status/E407.html