This is the problem which many of sharepoint designer usualy can't solve,
once you click the “Export” button once, other controls will become unresponsive since WSS post-back will not work. You cannot export another copy neither. The following property assignment of the “Export” button is required for other post-back to work (including the “Export” button itself)
So you have to just provide a OnClientClick property of the button, See below..
oBtn_Export.OnClientClick = “_spFormOnSubmitCalled = false;_spSuppressFormOnSubmitWrapper=true;”;
