Sept 11, 12:22 UTC, icinga reported tilerator not responding on all maps eqiad servers. Problem was solved by restarting tilerator on all nodes, this was not sufficient. A few requests seemed to be stuck on postgresql (I did not copy the list, I should have). Restarting postgresql on maps1001 (master) finally solved the issue.
The postgresql logs show processes waiting for lock (see below for extract).
2018-09-11 12:29:03 GMT [17900]: [3-1] user=tilerator,db=gis,app=[unknown],client=::1 LOG: process 17900 still waiting for AccessShareLock on relation 5134061 of database 16385 after 1000.061 ms at character 352 2018-09-11 12:29:03 GMT [17900]: [4-1] user=tilerator,db=gis,app=[unknown],client=::1 DETAIL: Process holding the lock: 9576. Wait queue: 29765, 26796, 29953, 22070, 13992, 13998, 14015, 14085, 14423, 14424, 14451, 14518, 17900. 2018-09-11 12:29:03 GMT [17900]: [5-1] user=tilerator,db=gis,app=[unknown],client=::1 STATEMENT: SELECT * FROM ( SELECT osm_id, way, admin_level::SMALLINT, (tags->'maritime')::INT AS maritime, CASE WHEN tags->'disputed' = 'yes' OR tags->'dispute' = 'yes' OR (tags->'disputed_by') IS NOT NULL OR tags->'status' = 'partially_recognized_state' THEN 1 ELSE 0 END AS disputed FROM admin WHERE COALESCE(tags->'maritime', '') <> '1' AND ( ( admin_level = '2' AND z(3.40282e+38) >= 2 ) OR ( admin_level = '4' AND z(3.40282e+38) >= 3 ) ) AND COALESCE(tags->'left:country', '') <> 'Demarcation Zone' AND COALESCE(tags->'right:country', '') <> 'Demarcation Zone' AND way && ST_SetSRID('BOX3D(-3.402823466385289e+38 -3.402823466385289e+38,3.402823466385289e+38 3.402823466385289e+38)'::box3d, 900913) ) admin_ LIMIT 0