I use Proxy Switchy! in Chrome, which sets not only Chrome's proxy settings but also "System Proxy" settings, i.e. Internet Explorer proxy settings (no pun intended!). This is useful because when I have to use Tor, all of my "system setting" applications will follow suit.
I know that Greenshot is using system proxy settings because when those settings point to Privoxy/Tor, Imgur upload gives this exotic error message. Whether I check "Use System Proxy Settings" in Greenshot or not, Greenshot always uses the system proxy, so I have to use a highly inconvenient workaround where I go to Chrome and disable the proxy every time I want to upload a screenshot to Imgur.
Upload breaks whenever proxy is on.
Change your system proxy settings, whether Tor/Privoxy/Proxy Switchy! or just directly from IE (maybe just set a bogus server). If you used Tor, try uploading to Imgur. Else, use Process Explorer to track TCP/IP connections, and you will see the incorrect connections.
Simply change the connection protocol (no pun intended again) to honor proxy settings (or the lack thereof).
Microsoft Windows 7 64-bit (quite behind on updates, but I don't think it should matter that much).
Google Chrome "Version 35.0.1916.153 m"
Latest Proxy Switchy! extension (http://goo.gl/O0eoBH)
Privoxy to forward HTTP localhost:8118 to Socks4a localhost:9050
Internet Explorer 10