Oracle SQL - illegal zero-length identifier errors

Need help with the below SQL. This SQL executes fine in SQLPlus. While executing from Peoplesoft application, we are receiving fatal SQL error randomly.

Oracle 11g (11.1.0.7)

OS - HP-UX PA_RISC 64 bit (ver - 11.31)

SELECT DISTINCT (SELECT g.oprid
                   FROM psoprdefn g
                  WHERE g.emplid = f.emplid AND g.oprclass <> 'WEB') emplid
           FROM sysadm.ps_rte_cntl_ln@fs9prod a,
                sysadm.ps_rte_cntl_uservw@fs9prod b,
                sysadm.psoprdefn@fs9prod f
          WHERE a.rte_cntl_type = 'PROJECT'
            AND a.rte_cntl_profile = b.rte_cntl_profile
            AND b.rolename = 'PRJ1_APPROVER'
            AND f.oprid = b.oprid
            AND