No shorts at night in the main dining room - resort casual is fine, but not shorts.
In fact, one night we were outside the main dining room of the POA and an older man wandered in wearing only his bathing suit - no shirt, shoes, etc. Staff met him at the doorway and quickly escorted him out. I think we was disoriented, but they got him right out of there.
If you want to wear shorts, you can have dinner in the buffet.