S9Y Smarty Error: unable to read resource: "404.tpl"
Most probably a preconfigured Serendipity installation moved to another Server. If Serendipity was installed on another host or another path on the same host and than moved without running the installer again a wrong path will be stored in the database. Update the path with the following SQL statement (replacing the path of course):
UPDATE serendipity_config SET value = '/new/htdocs/path/' WHERE name = 'serendipityPath';
Note: The table could have another name if you changed the table-prefix during the initial installation.