cache lookup failed for attribute 1 of relation XXXXXX

Hi, we have a logical backup process that runs every night since 5+ years.
It is a logical backup we use to restore a non production environment. We
use pg_dump in parallel mode in directory format.
Postgres version is 9.6.6

Tonight schedule failed with the following error:

pg_dump: [archiver (db)] query failed: ERROR: cache lookup failed for
attribute 1 of relation 2223152859

pg_dump: [archiver (db)] query 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, i.indisreplident,
t.relpages, c.contype, c.conname, c.condeferrable, c.condeferred,
c.tableoid AS contableoid, c.oid AS conoid,
pg_catalog.pg_get_constraintdef(c.oid, false) AS condef, (SELECT spcname
FROM pg_catalog.pg_tablespace s WHERE s.oid = t.reltablespace) AS
tablespace, t.reloptions AS indreloptions FROM pg_catalog.pg_index i JOIN
pg_catalog.pg_class t ON (t.oid = i.indexrelid) LEFT JOIN
pg_catalog.pg_constraint c ON (i.indrelid = c.conrelid AND i.indexrelid =
c.conindid AND c.contype IN ('p','u','x')) WHERE i.indrelid =
'2223152859'::pg_catalog.oid AND i.indisvalid AND i.indisready ORDER BY
indexname

We attempted to run the backup manually the 2nd time just after a couple of
minutes and it suceeded with no issues, the restore of the non production
env suceeded too. Not sure what this error is though. Have never seen it
before.

Index with that indexrelid does not exists

SELECT * FROM pg_catalog.pg_index WHERE indexrelid = '2223152859';
(0 rows)

Any idea about what happened? In the server logs I see only one occurrence
of that error and it is related to the failed pg_dump:

2018-07-19 01:04:26 GMT [127.0.0.1(52498)] [50816]: [13-1]
db=cmdv3,user=postgres ERROR: cache lookup failed for attribute 1 of
relation 2223152859

Thanks much in advance