I have a similar problem here. Opera told me that the page I was sent to after filling in my credentials didn’t exist. It happens —amongst other reasons-- when you disallow Opera to send referral information (Look under your quick options with F12).
I have a moral issue with the need to have the “send referral information” option turned on; not a single website really needs to know what other site I came from prior to visiting them. It’s not that they cannot function without it. If your site is that bad, close it. So I have it turned off by default.
To use referral information for transfers within the same domain is……….. and I say this as webmaster myself…… it’s odd. I don’t know any other website that does it.