bbPress.org

are you using a child theme ?? with bbpress.css in your child theme??

cause what i might think is that you have put bbpress.css into your child theme a long time ago

but the bbpress plugin updated their css with a repsonsive layout and you just dont have that code in your child themes bbpress.css

if im right , you can just check out the bbpress plugin and copy styles to your child themes bbpress.css