I'm also experiencing the same issue with 1.7.10
I fear that this is intended behaviour. They lock out hundreds of Minecraft users for the sake of f***ing security.
I'm not just speaking of students or workers who want to play Minecraft, even if restricted / not allowed by their authorities.
I'm speaking of those, who are forced to use proxies by their home internet providers to access the internet at all!!!
Thank you Microsoft
Sry I mean Mojang
Notch would never have done this!
Vote for #netneutrality
Edit:
OS: Win 8.1 x64
Java: 1.8.0_25 x64
It is a security and maybe also a money issue. Behind proxies you can of course spoof IPs, locations, override restrictions (as mentioned) and more, maybe even the login system can be tricked. It's a fact that this is not a bug, because overriding the commandline proxy option must be programmed by hand an be hardcoded in Java...