I’ve bypassed the error by commenting line 26 in /lib/pkp/classes/submission/reviewer/form/ReviewerReviewStep1Form.inc.php
$this->addCheck(new FormValidator($this, 'privacyConsent', 'required', 'user.profile.form.privacyConsentRequired'));
I understand that is not ideal to modify the core, and I’d still like to know why the error is being produced.