Interesting... I've been getting this error on logins since I first started using Piwik last year, since version 1.8, and I just ignored it because I've seen it with other apps, where cookies get confused or something.
It happens when I restart a browser session, and go back to my saved Piwik window/tab... it has logged me out, and has the full URL on whatever page I happened to be on at the time.
The error appears above the login fields, and if I try to login with the error displayed there, it doesn't log me in, and just returns me to the login page with the error above the fields, but if I clear the options off the login URL, say change the URL from
to just be
the error panel disappears, and the superuser login works just fine. And yes, I get it when using both Chrome and Firefox.
I just figured this was a known bug being worked on, since it happens to me all the time whenever I quit and restart my browsers (which are set to restart back to the previous session's windows & tabs)