The server returned a non-specific error when trying to get data from the data source. (SharePoint Designer 2007 error)

If you see this error in SharePoint Designer (click on Data Source and Show Data) then check the following:
1) If any calculated columns are used on corresponding list then make sure that any conditional statements using IF does not use "" in non-string data type fields. For example using IF([IsActive] = 'Yes', '', 'Test') - use of '' should be replaced with empty string if type is number etc
2) Check the SelectCommand within the Data View as well as DataFields and ensure that there are no fields/columns used which does not exist on the corresponding list