Queriy results erratic on Linux

Hi
My application runs fine on Windows.
However when I copy the files to the Linux server some queries return no
results or errors even though the records and tables exist!
It is always the same records or tables that are not found!
In case of a table the error is: function.pg-query: Query failed: ERROR:
relation "sites" does not exist in ....

Any idea what might cause the problem?

The server configuration:
    PHP Version 5.0.4
    PostgreSQL 8.1.4 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.2.2
20030222 (Red Hat Linux 3.2.2-5)

Thanks