Extension: https://chrome.google.com/webstore/detail/windscribe-free-vpn-and-a/hnmpcagpplmpfojmgmnngilcnanddlhb?hl=en
With SSL scanning enable (default option), loading any website with the extension enabled and proxy loaded results in ERR_SPDY_PROTOCOL_ERROR in the browser.
Disabling the feature fixes the issue.
This started happening in the last several days, so it sounds like a botched definition update.