This issue arises from time to time with a proxy that is poorly configured. If you get it once, just kill the tab and move on. If you get it regularly, we recommend that you contact your proxy provider and let them know the proxy IP address you used and the site it failed on.
We believe this error is sometimes shown from a proxy simply being too slow as well, so, again, your proxy provider is whom you should talk to have the issue rectified.
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