[kh99]

I'm not a php5 expert. But, I see that mysql_query() can return FALSE in the case of an error, and since the error message says that $query is a boolean, I'd assume that's what's happening. You could just check for that in the 'if', like maybe:

if ($query !== FALSE && mysql_num_rows($query)) {

but I don't know what the error would be or what you would want to do in that case, so that may or may not be the way to handle it.

The mysql interface has been deprecated so you should change to one of the other interfaces, but you can ignore that for now.

ETA: Actually, if that plugin is depending on the database connection that vbulletin opens, and you've changed to using mysqli in the config.php, then the error could be that there's no database connection. In that case what you probably want to do is rewrite the plugin to use the vbulletin database functions (which isn't as difficult as it may sound since the functions are named mostly the same as the mysql functions). That will also take care of the deprecated interface issue.