I also got this issue (which was previously closed as fixed), with a very long error message that had lots of red text overlapping other areas on the page.
This error message was observed after typing an incorrect password once, and then a correct password. Some strings have been replaced by [stringX] for privacy, and a space has been added after each protocol to avoid autolinking. I was attempting to log in with my Stack Exchange account that I was already logged into on most other Stack Exchange sites.
The return_to parameter does not have the same protocol (HTTP vs HTTPS) as the "actual URL" and the latter also has a whole lot more in parameter information.
Unable to log in with your OpenID provider: