Created Issue: this._sessions is null [27355]

I have a project which was working OK with the previous version of the toolkit. After upgrading through NuGet (uninstall and install again) I get this JavaScript error in the browser (FF18 and IE10).

Error listed in FF's Error console:
Timestamp: 2/6/2013 3:58:53 PM
Error: TypeError: this._sessions is null
Source File: https://10.10.10.10/XXXXXX/ScriptResource.axd?d=66ZXgN18tFnfK0tcq8415ZrtBpChbiNEHSvgOXaqbvivgPiIge3W5tWyy9TWh2gN-Xm6QHs8NdtePozmS-TsbGDAdW3OWlSeMtWwwbNs-bbb7Yq0KJFAd_2xTRjfiyejavhspCXAM6O2uYyHqe0Saw2&t=48e38e19
Line: 369

Clicking on the link it opens the file:
// Name: MicrosoftAjaxWebForms.debug.js
// Assembly: AjaxControlToolkit
// Version: 4.1.7.123
// FileVersion: 4.1.7.0123

And the Line 369 says:
__this._sessions.push(session);__

Checking the source the same code appears in ScriptLoader.js line 44.

The .NET FrameWork on the server is 4.0, and I'm working with VS Express 2012.