When I try to log in to Stack Overflow from Opera, it sends me to the default home page without logging me in. It isn't complaining that my password is wrong, it just won't log me in. It works just fine in IE.
I'm using Opera 53, and I can use it to log into Stack Exchange just fine.
Steps I have taken is to clear my temporary internet files, including my cookies and cache, on Opera 53. I do not have access at work to change DNS settings, but since IE is working fine, I'd assume they are working fine too.
Edit: I just noticed that I can see my Stack Overflow inbox from Stack Exchange, but when I get there it is not available, as I am not logged in there.