Class 'HttpClientOAuth' not found after logging in at authentication server

After logging in at LinkedIn I am returned to my client website and get this error:

Fatal error: Class 'HttpClientOAuth' not found in /Users/erik/www/vngkickstarter/htdocs/sites/all/modules/contrib/oauthconnector/oauthconnector.module on line 91

This is what I have found:
- The http_client_oauth is enabled.
- It happens independend of PHP-version (tested 5.4, 5.6)
- Adding the following line fixes the error:

module_load_include('inc', 'http_client', 'includes/auth/oauth/HttpClientOAuth');

or

module_load_include('inc', 'http_client_oauth', 'HttpClientOAuth');

So, it looks like something goes wrong with Drupal's autoloading.