Since this is working just fine without the AJAX configuration it should be some AJAX setting which is missing. I'm puzzled however with the note about the AJAX panel containing all the controls. Have you tried wrapping the whole UserControl in an AJAX Panel resizing in the hosting page?
Otherwise, I'd suggest you put RadAjaxManager in the hosting page along with RadAjaxManager proxy in the user control and configure the proxy with the necessary relations.
If you still cannot resolve the problem we will have to look into and debug your exact problematic user control. Please submit a formal support ticket through your Client.Net account where you can send a runnable sample. We will gladly investigate the issue and get back to you with our findings/resolution.
the Telerik team
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