pg_upgrade fails: "Mismatch of relation OID in database" - 9.2.4 to 9.3.2

I've tried to upgrade a large production database from 9.2.4 to 9.3.2
using pg_upgradecluster (using pg_upgrade method) and it fails with the
error below. Upgrade using dump/restore seems to work on a test system
but would require a too long downtime on the production system.

# pg_upgradecluster --method=upgrade 9.2 main /data/postgresql/9.3/main
[...]
Removing support functions from new cluster ok
Copying user relation files
/data/postgresql/9.2/main/base/12034/7174572
Mismatch of relation OID in database "rg_www": old OID 22046, new OID 17671
Failure, exiting
Error: pg_upgrade run failed
[...]

Any thoughts? How can this be fixed?

Regards, SW.