Visiting https://en.wikipedia.org/w/api.php?action=query&meta=notifications gives me:
<error code="internal_api_error_MWException" info="Exception Caught: Internal error in ApiFormatXml::recXmlPrint: (list, ...) has integer keys without _element value. Use ApiResult::setIndexedTagName()." xml:space="preserve" />
https://en.wikipedia.org/w/api.php?action=query&meta=notifications&format=jsonfm works just fine.
Version: unspecified
Severity: normal