theres a large number of ways this can happen (see http://www.google.co.uk/search?hl=en&q=Page+cannot+be+null.+Please+ensure+that+this+operation+is+being+performed+in+the+context+of+an+ASP.NET+request.&meta= ) for some examples. I'd suggest it might be a good idea to disable the updatepanel as it often then shows the "real" error. To do this set EnablePartialRendering="false" on the scriptmanager control - if you're using the automatic scriptmanager injection in dotnetnuke you'll have to set this in your page_init of the module.

Cathal

disabling partial rendering disables the ajax, which allows you to better debug your code. If the server event is not triggered, it's because your code is incorrect for some reason i.e. the button you're pressing may not be a server control (or have runat=server), or you may not have the server event wired up correctly (if using vb.net), or may not have a delegate defined for the event (c#).

Cathal