Crack, sure, or tin whisker is another possibility.
Or leaky flux or other mildly conductive residue, which can form tin tracks by electrolysis, particularly in humid climes. I would guess a VFD might be made with conformal coating, which slows that sort of thing, but maybe doesn't perfectly prevent it.
If it's external to the component, removal, cleaning, soldering, and cleaning again, should be good for a long while. If it's internal, do all that while also installing a brand new component (you know, from a reputable distributor and all that).
Tim