Have a look at: http://stackoverflow.com/questions/4599696/use-pg-restore-to-restore-from-a-newer-version-of-postgresql, this might be the same issue and the solution is posted here.
In Mendix hosting, you can restore .backup files that are created with the pg_dump program from either PostgreSQL 8.3 or 8.4, not 9.x yet.
I recommend using 8.4 locally.