https://github.com/nextcloud/user_saml/issues/208

Edit this file:
/var/www/html/nextcloud/apps/user_saml/lib/Controller/SAMLController.php

search for @NoSameSiteCookieRequired and edit this line:

/**
         * @PublicPage
         * @NoCSRFRequired
         * @UseSession
         * // @OnlyUnauthenticatedUsers
         * @NoSameSiteCookieRequired
         *
         * @return Http\RedirectResponse|void
         */
        public function assertionConsumerService() {
                $AuthNRequestID = $this->session->get('user_saml.AuthNRequestID');
                // if(is_null($AuthNRequestID) || $AuthNRequestID === '') {
                //       return;
                // }