Error on play but compiles fine

This is a bit hard to explain.

Let’s say I open my project in a freshly opened UE4 editor instance. If I hit play it will complain one of my BPs has an error and won’t compile (and I can choose to go ahead or not)… The issue is that when you go into the BP it compiles fine. At no point is there a reference to this error

And this wouldn’t be so bad but I am sure this is causing an issue when I build the game, as it is as if the BP is not executing (although the components appear in the level)

How would I track down the bug or error?