Seasons came after Pets..so you use the latest hacks for that EP.
Remove all your hacks and update them for Seasons...most mods and hacks do have updates..that should fix your problem, I hope.
But you should get the Sims2 Hack Conflict detection program from here:
http://simwardrobe.com/It will spot any conflicts in your hacks before you run the game..just put it in your downloads folder by itself.