rastaferraille you need to access your forum via the url defined in config.php. It has to be the exact same domain (including www if it's there, or without it if it isn't) and exact same protocol (http or https, preferably use the later in the config).
To prevent users from using an invalid url you can setup redirects in your webserver config, or use the Canonical Url extension to perform those redirects.
On urls that aren't the one from config.php, this will cause assets and API to be fetched through a different url, which cause cross-domain errors.
