Hello,
I think this error has no direct relation with JoomSEF; it may be some problem of how the files are loaded by Joomla.
I have checkd the source in the file you have mentioned in the error and the lines there are:
jimport ('simplepie.simplepie');
if(!is_writable(JPATH_BASE.DS.'cache')) {
$options['cache_time'] = 0;
}
$simplepie = new SimplePie(
$options['rssUrl'],
JPATH_BASE.DS.'cache',
isset( $options['cache_time'] ) ? $options['cache_time'] : 0
);
So as you can see, there is jimport function called that should load the SimplePie class.
Therefore I do not see, why the class cannot be loaded.
Revealing more about this would require debugging step by step. I am just afraid this is already out of range of the support we can provide here.
If you would be interested, we can offer you a
paid support.