Initially I logged in using my Google account. Later, I also connected my Facebook account to my Stack Exchange account. Now I am unable to login to the site by using any credentials.
It seems something is doing a MITM attack. It can be your corporate firewall or proxy which wants to know what you are typing in, but it can be something else too.
If you are within a company network, and the
10.221.50.7 machine is a corporate firewall or proxy, you know you have to knock on the system administrator's door. Else it might be a plugin or your ISP fiddling with your request.
I would suggest to try another browser, another machine and another network to check who is at fault here.