Hi, @emad said in Nginx "404 Not Found" Error after POST action to "$PORTAL_ACTION%2quot;: So, do I miss knowledge with CP behavior or should I modify something? You might as well check (== read and understand what happens when and why etc) this file /usr/local/captiveportal/index.php It's the file that's get 'executed' when a users is redirected that the portal login interface. Read that file carefully - and also check this one : /etc/inc/captiveportal.inc which contains all the functions. For instance, you'll find why/where/when a string like $PORTAL_ACTION$ is replaced by the correct URL before getting send to the client. Throwing $PORTAL_ACTION$ at the client's web browser who throws it back to the captive portal web server will produce 404 errors.