I get the following error 'Warning: main(./libs/smarty/Smarty.class.php): failed to open stream: No such file or directory in *.php on line *' or 'Warning: main(*.php): failed to open stream: No such file or directory in /*.php on line *'. Help please

This problem is due to an Upload error, that you certainly didn't notice, because you didn't check your FTP AHAH! You think everything went ok but it didn't... Well, this is not a big deal. Simply reupload all the files, and this time... check that all files are correctly uploaded.