pg_dump fails with the following error: "ERROR: cache lookup failed for index 1531353157"

Hello,
 
The server is still running but pg_dumps output the following error. What should I do?
 
Thanks,
Benjamin
 
OUTPUT:
 
pg_dump: SQL command failed
pg_dump: Error message from server: ERROR:  cache lookup failed for index 1531353157
pg_dump: The command was: SELECT t.tableoid, t.oid, t.relname as indexname, pg_catalog.pg_get_indexdef(i.indexrelid) as indexdef, t.relnatts as indnkeys, i.indkey, i.indisclustered, c.contype, c.conname, c.tableoid as contableoid, c.oid as conoid, (SELECT spcname FROM pg_catalog.pg_tablespace s WHERE s.oid = t.reltablespace) as tablespace FROM pg_catalog.pg_index i JOIN pg_catalog.pg_class t ON (t.oid = i.indexrelid) LEFT JOIN pg_catalog.pg_depend d ON (d.classid = t.tableoid AND d.objid = t.oid AND d.deptype = 'i') LEFT JOIN pg_catalog.pg_constraint c ON (d.refclassid = c.tableoid AND d.refobjid = c.oid) WHERE i.indrelid = '1531340182'::pg_catalog.oid ORDER BY indexname