If I were you, I wouldn't worry too much about planarity problems. OK, 11.3σ is 11.3σ, but even 25σ relates to only a small atomic displacement. If you are a crystallographer, well, fix this problem, but don't expect any more advantages than a shorter WHAT_CHECK report, and if you are an end-user, start worrying somewhere beetween 25σ and 50σ...