I get the form just fine, fill in the field just fine, goes back to the previous page (assuming guest can view it) and I'm not logged in. As of 9648 all was fine.
Update: it's possible that it may have been a silent failure due to an inability to create a new session file, as /tmp was quite full at the time of the problem. I will test this further.
There's another possibility. If the host of the page that started the login process is different to the host of the page actually ended up at after the login, then the cookie can get assigned to the wrong host. For example, if you are on a view page on
http://localhost, and you click "login", then the page opened will have the URL
http://whats.in.localsite.for.url.host/... The cookie is set in the browser for this host, but the "original url" is still
http://localhost.
Anyway, there has been no meaningful follow up on this, so I'm closing it. i have added a whole bunch of tracing to try and hel you debug if it happens again.
CC
Renaming summary for release notes.
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SP