So it is very unlikely that the issue is the diary. Aman plays a key role in the misunderstanding between AarYa.
He's an old friend of Aarti's and maybe even an ex boyfriend. We don't know. However, the fact of the matter is that Yash feels Aarti has betrayed him and it's connected to Aman.
This can be one of the following:
1) The Prashant truth. I don't feel like this is the reason because it seems too early, and there hasn't been any promotion about it.
2) Yash thinks Aarti/Aman have a thing going on. He thinks Aarti may have cheated on him. I think this is what's happening because of a few things. Think back to yesterday's episode. Yash told Aarti "I have no rights to punish you so I'm punishing myself". Why would Yash tell Aarti that he has no rights to punish her? Because he claims he did this Punar Vivah for their kids and he never accepted Aarti as his wife in his true sense. He realizes his feelings for her because he is so affected by her. This is why he's punishing himself. He thinks that he has fallen in love with a betrayer, and it hurts him to think that he got over his first wife only to fall in love with a traitor. That's why he smashes Arpita's pictures: because he thinks that he should have never moved on. He was happier in his earlier miserable state. I feel like this is the reason that Yash is behaving this way.