Anyone can claim anything. For instance, I'm a millionaire. Honest.
I agree that the problem would be structural, though. It just seems a peculiar problem to have in context.
My first guess would be AoA and/or camber. Ferrari asks a lot more of its rear wing than does Red Bull. The main plane is much more cambered, and the flap is set at an appreciably higher AoA.
Silverstone 2014
Maybe they've gone a bit too far in trying to maximize downforce under the new regulations. Last year's Silverstone wing didn't appear nearly as aggressive, despite rules that allowed for more surface area and for the assistance of a beam wing.
Silverstone 2013
EDIT: I just realized that angle of attack and camber are probably the most "Thank you, Captain Obvious" reasons for why any wing might stall. Nonetheless, the above is an excellent representation of my obviously exquisite(ly absurd) insight into the mysterious world of aerodynamics. If anyone would like for me to elucidate the wetness of water or why women always be shoppin', just let me know.
EDIT for the sake of brainstorming: Maybe...just maybe
my cockamamie idea about reducing drag with the airbox wing wasn't far off the mark, even if it was
completely wrong.
Quick recap with broad strokes: the airbox wing, circled above, appears to be cambered to produce lift rather than downforce. If so, tip vortices from it will rotate opposite to those shed by the rear wing. I'm pretty sure that's correct regardless of any further implications. As for those implications...
1. What happens when/if those opposing sets of vortical flows interact? Will they sort of cancel each other out, or will they merge to form larger vortices?
2. Is it possible to direct vortical flow from the airbox wing to the underside of the rear wing to "energize" flow? If so, could that allow the use of a larger AoA on the rear wing? How difficult would it be to manage (might the rear wing stall if it goes wrong)?
3. The roots of the airbox wing are sharply angled down, presumably to direct air flow over
cooling inlets on top of the engine cover. Could it be possible that any interaction between the airbox wing and the rear wing is a completely unintended side-effect of this cooling solution? If so, what could cure it?
4. A totally unrelated attempt to appease Bill Ockham: maybe the slot-gap just got clogged with something. --- looks pretty dirty, yo.