Controls on ASCX widgets not rendered correctly (SF 4.4)

Hi all,

We've got a strange issue while rendering controls in custom ascx widgets.
On some pages we cannot change control properties from code-behind.

For example, we have a page with a simple asp:panel and asp:label.
In de code-behind we change the panel's 'Visible' property to 'true' and the label's 'Text' property to "Hello World!".

While debugging the rendering of the page we can actually see these properties being changed in the code-behind. But the page that's actually rendered in the end does not reflect these changes. Even stranger; on SOME pages it works, and on some others it won't.

Deleting the affected pages and re-creating them has not effect.
We've tried disabling all output cache settings in sitefinity. But that didnt change this strange behaviour. 

Any idea's what is causing this issue?

Regards,

Erik

1) Is the viewstate enabled in the properties for the page?
2) Can you paste the code in here?