problem using AJAX 3.5 (Dev ok, live not ok)

My web-app using .NET 3.5 and AJAX 3.5, is working well using the local
machine.

When running on the webserver (windows 2003 + .NET 3.5 installed) the page
works just fine, until I hit 'Next' wich is supposed to do a postback (in
the updatepanel) and show the next UserControl (it is sort of a wizzard like
app). Then I get 'Error on page.' in the bottom-left corner of IE and this
message :

Sys.WebForms.PageRequestManagerServerErrorException: Object reference not
set to an instance of an object. (Line 605, char 13)

I tested this using Vista + IE7 and XP Pro + IE7.
I did not write any kind of javascript in these pages/usercontrols and am
really surprised about this.

If I turn off PartialRendering I get the following error during the loading
of the page :

'Sys.WebForms.PageRequestManager' is null or not an object (Line 925, char
5)

I really don't know what to make of this, I thought Ms-AJAX was just
supposed to be drag-n-drop AJAX.


Can anyone help me out, I'm moving toward my deadline really fast...


TIA,
Jurjen.