According to this issue tracker, this was an on going issue and it is supposed that is fixed. However, it seems like some customers are still facing it.
I recommend you to run pg_dump on your own against your instance (without using Cloud SQL export) so that you be able to export your Cloud SQL database.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…