After trying everything for almost two days, we've decided to just hack a solution.
- a page based on a master page
- a user control called by the page dynamically.
The user controls contains RadGrids, RadEditor, etc.
If you take the code (of the master page, page and user control) and put it in a single page (as test) all works fine.
That means the problem is due to the order of Ajaxifying in a page + user control.
In short, what we ended up doing is:
In the user control's code behind we added the following in the Page_Load:
If Page.IsPostBack = False Then
If Session("1st") IsNot Nothing Then
This reloads the page on first load.
It's not very elegant, but it sure saves time.