Summarized answer since this seems to be a common problem:
This is due to a bug in MySQL 5.1.46 and 5.1.47:
(The LEFT JOIN using a condition that is definitely false (=NULL) makes the results empty, when it should simply create a NULL column.)
For now the solution is to change this line in qa-db-selects.php:
$selectspec['source'].=' LEFT JOIN ^uservotes ON ^posts.postid=^uservotes.postid AND ^uservotes.userid=$';
... to ...
$selectspec['source'].=' LEFT JOIN ^uservotes ON ^posts.postid=^uservotes.postid AND ^uservotes.userid=COALESCE($,0)';
The next version of Q2A will work around this a different way.