Form saves not working since upgrade to 4.0

[RubenSimoens]

We just upgraded to a 4.0 nightly for the first time
and it broke all of our forms saves:

editForm.saveData(new DSCallback() {
  @Override
  public void execute(DSResponse response, Object rawData, DSRequest request) {

The save happens but get a javascript exception:

Uncaught JavaScript exception [TypeError: _70 is undefined] in http://127.0.0.1:8888/slims/sc/modules/ISC_DataBinding.js, line 1070

More specific here:

var _30 = function(_49, _69, _70, _71, _72) {
	var _31 = _70.criterion;
	var _32 = _70.record;
	var _33;
	...

I can't really reverse engineer what's missing here. Do you have any pointers about what could be missing? The error only happens when validation succeeds.

EDIT:
This apparently also happens on grid saves


This is on:
(SmartClient Version: v9.0p_2013-12-06/PowerEdition Deployment (built 2013-12-06))
GWT 2.5.1
Firefox 25.0

[RubenSimoens]

Nevermind, found the issue