Forms 12 - Error ORA-06508 opening a form

I am facing a strange ORA-06508 on opening a form (Forms 12.2.1.3.0)

Form have an attached PLL library. Error is raised when WHEN-NEW-FORM-INSTANCE trigger calls some procedure stored in PLL library. But it occurs only at first form call; if I redo the same operation, or open another form before, it works correctly.

Furthermore, if user logging on application has schema owner privilege, error does not occur.

how can I fix it? thanks for help