On break: I am on my break now but she is woking. (Discontinuation from work or activity ) At break: we can gossip about her during/at break. I suppose it implies specific location or period of time.

Thank you, Chris.

You would never use “at breaks”, you’d only ever use “on break”. For example; I’m on break right now. - meaning you’re on break at work. “We’re on a break right now.” - meaning your relationship with someone is on hold