The following exception occurs if the system fails to find a specific custom site definition on the server:
Microsoft.SharePoint.SPException: Failed to instantiate file "default.master" from module "DefaultMasterPage": Source path "default.master" not found. --->
Make sure that all the onet.xml files have been correctly deployed on the 12HIVE (this happens if the deployment package either doesn’t include the necessary onet.xml files or the solution manifest doesn’t contain entries to the corresponding onet.xml files).
If the system doesn't find the onet.xml, it will try to create the site from the native definition.