It looks like the cabinet corner itself has come apart and the split is a symptom. What does the outside look like?
I think you need to use Gorilla Glue and large clamps (or a band clamp) to close up that split.

Alan

I've used both Titebond wood glue and the urethane Gorilla glue (original one that needs moisture and then it foams up). The Gorilla glue is great stuff, extremely strong on wood. I also use it when building fence gates.

Either one is good.

Alan.