This error drove myself and a colleague up the wall recently. We were trying to do a simpel file copy in SSIS 2005 from a network location to a folder on the machien running the package. We were laoding the source and destination at runtime from a table and setting the appropriate ssis properties via expressions.
We kept on getting the “Could not find part of the path” error complaining about our UNC source. Eventually we realised the destination path was not valid as a folder in the path did not exist. This error was misleading as we spent a lot of time checking permissions etc on the source path but not the destination.. where the problem actually was!