I had the same issue, default queues were empty and the PHP shows fatal error:
Fatal error: main(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "QueueSort" of the object you are trying to operate on was loaded before unserialize() gets called or provide a __autoload() function to load the class definition in /opt/bitnami/apache2/htdocs/osticket2017/include/staff/templates/queue-tickets.tmpl.php on line 72
Eventually one or other queue shows up, but most shows the error.
I commented out the line 72 on this file, now it looks like:
// $sort['queuesort']->applySort($tickets, $sort['dir']);
After that the queues worked just fine. I didn't miss any functionality - I guess.
I though this was some issue in my server (I'm running a bitnami virtual machine).
I don't understand the OSticket source code, so I'm not sure what are the consequences of commenting this line.