@-flo-:
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For sake of user’s saved login credentials I would prefer to have my single existing zone use ID 0 so the login page is still to be found on port 8000 as before. Is this possible, if so how?
"Login credentials" are not related to "using port x to authenticate" - "what is the zone ID (because many can exist)".
So, in case of doubt, just wipe all portal settings.
(go even to ssh, start viconfig - wipe everything between <captiveportal>and</captiveportal> , save back, reboot pfsense and set up portal up again.
(or export config - edit file with good editor like notepad++, and import back in - and then setup your portal again.)
The fact that it uses a port like 8001, or 8002 (https) or whatever is just a "behind the screen pfSense" thing.