It's likely the new animations causing instability in Ren'Py, try this patch:
https://f95zone.to/goto/post?id=3457674
It's a small bonus scene (with no sex) after 'The End' screen & credits - every ending has one, you probably exited back to the main menu too soon!
Also, double check if you were actually playing the Karen+Nikki ending route (you know you're on the right route when you get the Professor sex scene in the doctor's office then the Karen+Nikki threesome in the dorm room), I don't use the mod but read some people have had problems with it...
And to anyone saying they are using the guide but getting the wrong ending - clearly you weren't paying attention to it! The main thing to keep in mind about the choices in a Karen+Nikki playthrough is that you need to favour Nikki over Karen for both the magazine & bowling choices!