Your problem is that you are using a UDT. When you create a UDT, DB2 will automatically create the casting functions to the base datatype (integer if your example). These functions are in the same schema as the UDT, so to use them you need to fully qualify the function. Unfully qualified functions are searched in the "PATH" which, by default, is SYSIBM,SYSFUNC,SYSPROC,<CURRENT USER>.
Change your queries to fully qualify the cast function, like this:
SELECT * FROM case WHERE inst01.INTEGER(case_id) = ?
and
SELECT * FROM enrollee_state WHERE inst01.INTEGER(enrollee_id) = ?
HTH
Andy