Two ways: 1. remove the validation plug-in, 2. upgrade to the very latest validation plug-in code ( https://github.com/bigfix/jquery-validation/commit/9aa53f9241224ba349f9fe1b2c801be0f431b63b ).
There are quite a few discussions on the web about this issue with jQuery 1.5 and the jQuery validation plug-in ( http://www.google.com/#q=jquery+1.5+validate ). Basically it appears that the plug-in wraps and overwrites the $.ajax stuff, thus causing the problem since Ajax for an overhaul in jQuery 1.5.
Source: DataTables forum