Hi, Tyssen -
That shows up if Cookies only is set for the control panel, so it is related to those security settings.
Also, I see this: “You are not authorized to perform this action” - this can also happen if the IP has been banned -or a partial IP. If you’ve got an IP ban list going on, you might find out this fellows IP (My favorite way to do this is to google “what is my IP” - shows at the top of the search results) and double check that it is not, in fact, banned.
And you’ve tried with sessions only for the admin and user side? I know you played with cookies, just want to confirm that combination was tried.
Thank you!