The function getQueryString in class SMWNamespaceDescription (SMW_Description.php) does not handle NS_CATEGORY namespace accordingly (see , ). : is not set in cases namespace = NS_CATEGORY and creates a problem in queries selecting on NS_CATEGORY pages.
 «Category» namespace, a : is again needed in front of the namespace label to prevent confusion, e.g. [[:Category:+]].
PS: I looked at the code and it seems not to have any special handling of NS_CATEGORY, so this must be broken since. Sorry I do not have the time to fixed this myself, so I just make note of it.