I found this post at the Smarty forum
The problem seems to be a wrong or relative path to the plugin folder. After setting the path to the plugin folder absolut, it works fine, even with namespaces.
Have also a look at the Smarty documentation:
Technical Note
For best performance, do not setup your
$plugins_dir
to have to use the PHP include path. Use an absolute pathname, or a path relative toSMARTY_DIR
or the current working directory.