Could it be that setting cookies for the domain was disallowed via the permissions dialog? When I do that, I see behaviour like you are describing it: when I e

Could it be that setting cookies for the domain was disallowed via the permissions dialog? When I do that, I see behaviour like you are describing it: when I e.g. tried upvoting jedberg's comment with cookies disallowed, it actually worked. I was redirected to the login page and back to the discussion without staying logged in. After undoing the permission change, logging in and having a look at the exact comment, I saw that the vote actually went through. Posting a new comment showed the same behaviour. You can find that permission on the "page info" dialog (shortcut Ctrl+I if that isn't localized?)? Edit: the error message on the console would be different though. The described procedure leads to the message "Setting cookie ... has been denied because of a permission set by the user" (translated from German, so the actual message will be different). It looks like I was only describing the behaviour when cookies are unavailable and not the actual reason.