I had the same problem as some others here, with the httpclient class not being found. I got the error when getting the postback from paypal to the express.php file.
I added in an include line, following the advice from: http://www.oscommercecoders.com/fatal-error-class-httpclient-not-found-in-homexxxxxxxpublic_htmlmyshopincludesmodulesshippingups-php-on-line-265/
and now things seem to be happy.