Opera 34.0.2036.50 vs Firefox 44. on Windows 10.

With Firefox, I can login normally. But not with Opera

I've had similar issues with Opera from within Win10 when logging into other sites as well as this forum. Finally gave up the ghost and un-installed Opera; it's definitely the browser and not this forum specifically.

Please give some details about what happens. What error do you get? What dialogs do you see? What cookies are set?

Sorry for the very late reply.
I think I already found the cause.

When logged in normally, the link "Log in" located on top of this forum will be replaced with "Log out [username]".
In my case: "Log out [TmX]"

My Opera ocasionally switches its proxy, from not using proxy at all to 127.0.0.1 port 8118 (malware?)

That's why I have to revert the change, so I can log in.