I know this is an old post. I got the same error with ArcGIS Server 10 and PostgreSQL when joining a feature class to an SDE table. The solution turned out to be that when doing a join, the column names are prepended with the original table names, so my columns look like table1.field1 and table2.field2 instead of just field1 and field2. In the case of postgresql, I actually needed to append my database and schema names, so the outfields look like database.schema.table1.field1, etc.
Just wanted to share in case anybody else stumbles on this post like I did.
