Thank you very much. I am running Plex Media Server in a Debian server toguether with utorrent. I usually managed my utorrent server from a Debian client in my local network. Suddenly I got this "Invalid request" error. I do not know which the cookie giving troubles, but the exact workound RomanZ describes worked perfectly for me. Thank you for the support. In firefox you can go to History -> Clear recent history, then choose "everything" in time range, "Details" and "Cookies" only.