If you receive this then the fix is fairly simple.

You need to log into your site and from the site root navigate to
/plugins/obgrabber-adapter/content/fields

and edit the file named gruser.php
Line 13 (or thereabouts) will be an include which is now incorrect.

require_once JPATH_SITE . DS . 'libraries' . DS . 'cms' . DS . 'form' . DS . 'field' . DS . 'user.php';

Needs to be changed to

require_once JPATH_SITE . DS . 'libraries' . DS . 'fof' . DS . 'form' . DS . 'field' . DS . 'user.php';

Save and your done.