Hi Toon,
This cookie is set by Mendix in login.js. There are multiple possible solutions to this problem. You can customize login.js to set a different cookie, or overwrite the cookie using a nanoflow after login. I'd say the first is probably the better option. Follow the steps described here to add login.js to your theme folder, then find the part that needs to be changed. You will want to change location.pathname to “login.html”, to hardcode it to redirect to login.html after logout.
"originURI=" + location.pathname,