https://bugzillapreview.wmflabs.org/search/query/open-tasks/ gives:

Unhandled Exception ("HTTPFutureHTTPResponseStatus")
Unhandled Exception ("HTTPFutureHTTPResponseStatus")
[HTTP/400] {"error":"SearchPhaseExecutionException[Failed to execute phase [query], all shards failed; shardFailures {[kVa6pnbJSEyw1PlUSHyE1A][phabricator][0]: SearchParseException[[phabricator][0]: query[ConstantScore(+cache(BooleanFilter(_field_names:relationship.open.phid)))],from[-1],size[-1]: Parse Failure [Failed to parse source [{\"query\":{\"filtered\":{\"query\":{\"match_all\":{}},\"filter\":{\"and\":[{\"exists\":{\"field\":\"relationship.open.phid\"}}]}}},\"sort\":[{\"dateCreated\":\"desc\"}],\"from\":0,\"si...