This has just been reported on the users list. apa.layout has a bug in the spelling of 'Acknowledgments'. However, before I apply the patch, I suspect that lyx2lyx will need some modification.
Jose, can lyx2lyx (actually the lyx format for a stable version of lyx) handle such a change? Angus -Style Acknowledgments - LatexName acknowledgments +Style Acknowledgements + LatexName acknowledgements LatexType Command + Font + Size Small + EndFont + Margin Dynamic + LeftMargin MMN + RightMargin MMN + Align Block + AlignPossible Block, Left, Right, Center + Labeltype Static + LabelString "Acknowledgments: " + Labelsep M + LabelFont + Series Bold + Size Small + EndFont End -- Angus