Errors placing SmartPart on page


We tested the SmartPart a few weeks ago, and it seemed to work really well. We're now trying to use it to host a web user control that uses AJAX, but can no longer place either version of the SmartPart on a SharePoint page. When we try to select the SmartPart, we get "Unable to add selected web part(s). SmartPart: Exception occurred. (Exception from HRESULT: 0x80020009 (DISP_E_EXCEPTION))". Trying to use the SmartPart with AJAX gives this error: "Unable to add selected web part(s). SmartPart with AJAX: The web part you attempted to add no longer exists in the Closed Web Parts gallery. Refresh your browser to update the Web Part Page and the Gallery".
Has anyone else seem this? Can anyone point us toward a solution?