@leocg im not 100% sure how but the client js does not want to redirect with opera gx's default agent and @JessusChrist44 you can paste it in at the bottom from my post

@leocg not 100% sure but the client js file does not recognise opera gx's user agent and refuses to redirect

Pinterest is currently broken on Opera GX because the browser misidentifies itself and Pinterest serves incompatible scripts. The easiest fix is to install a user‑agent switcher extension and make Opera GX pretend to be ChromeOS. I recommend this one:
https://addons.opera.com/en-gb/extensions/details/user-agent-switcher-8/
Set the user agent to:
Mozilla/5.0 (X11; CrOS x86_64 10066.0.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/143.0.0.0 Safari/537.36
After switching, Pinterest will load normally again.