They are prone to honey fungus, and the cracks at the base could be indicative of the workings of that pathogen. However, before the death knell is sounded, let's check other reasons. Water lacking? Although they love the sun, they do need water when they're growing so fast.

Oops, sent that before I meant to. The other thing is that robinia and acacia both crave good drainage (which doesn't mean no water) and last year's wet may have set your tree up for some root damage. There is a study on at this moment re robinia 'Frisia' and its failure to leaf up properly after a cold winter

Thanks for those observations Worthy1. The extended cold winter may indeed be the 'root' cause!