unauthorized_client NextJS Frontend and Go Backend

Hello, as the title states. I have a SaaS with a golang backend with an API application, and a nextjs frontend with a PKCE application. The frontend can authenticate users normally and get the access and ID tokens, but when the access token is sent to the backend token introspection fails with the error "unauthorized_client". The same exact setup was working one day ago, what could have ruined it? I am on zitadel cloud.