I have the same problem after upgrading to 10.4.9:
2018-11-03 00:06:14 CET [41253]: [1-1] user=, db=, app=, client= ERROR:
found xmin 2502024502 from before relfrozenxid 150137048
2018-11-03 00:06:14 CET [41253]: [2-1] user=, db=, app=, client= CONTEXT:
automatic vacuum of table "postgres.pg_catalog.pg_tablespace"
2018-11-03 00:06:29 CET [41305]: [1-1] user=, db=, app=, client= ERROR:
found xmin 2502024502 from before relfrozenxid 150137048
2018-11-03 00:06:29 CET [41305]: [2-1] user=, db=, app=, client= CONTEXT:
automatic vacuum of table "postgres.pg_catalog.pg_tablespace"
2018-11-03 00:06:44 CET [41625]: [1-1] user=, db=, app=, client= ERROR:
found xmin 2502024502 from before relfrozenxid 150137048
2018-11-03 00:06:44 CET [41625]: [2-1] user=, db=, app=, client= CONTEXT:
automatic vacuum of table "postgres.pg_catalog.pg_tablespace"
2018-11-03 00:06:59 CET [41676]: [1-1] user=, db=, app=, client= ERROR:
found xmin 2502024502 from before relfrozenxid 150137048
2018-11-03 00:06:59 CET [41676]: [2-1] user=, db=, app=, client= CONTEXT:
automatic vacuum of table "postgres.pg_catalog.pg_tablespace"
your workaround helped, thank you
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