The form is validated when submitted: every field marked Mandatory must have a value, otherwise the visitor sees "All mandatory fields should not be empty"; a submission with nothing filled in returns "All fields are empty". "Unknown error in the form or in the page" means the form or page could not be identified — typically the module's Form option is empty or the form was deleted; re-select the form in the page module. Check your field settings in Form Builder and re-test with Preview.