The default DatePicker of Apex was throwing a security error when we were testing our Apex application. We had to pass a user/password for a call on the wwv_flow_utilities package.
Just registering that package as secure plsql as System Administrator in Apps solved the issue (table: fnd_enabled_plsql).
But when we changed the month in the DatePicker, the same error popped up again.
Seems Apex adds the schema name to the package call, and as such the secured plsql scan can not handle the call.
So we were forced to go for a custom DatePicker.
A pity we can not wait for Apex 4.0 were jQuery will be included by default. Demostuff.