[Lynne]

In order to get the full error, you need to view the source code on the error page. Doing that shows me this error:

 <!--
Database error in vBulletin 3.6.9:

Invalid SQL:

            SELECT * FROM mashupmb_user FORCE INDEX (lastactivity)
            WHERE lastactivity &gt; 1209168000 ORDER BY username;

MySQL Error  : Key 'lastactivity' doesn't exist in table 'mashupmb_user'
Error Number : 1176
Date         : Saturday, April 26th 2008 @ 05:08:09 PM
Script       : http://www.mashupmb.com/
Referrer     : https://vborg.vbsupport.ru/showthread.php?t=177244
IP Address   : 
Username     : Unregistered
Classname    : vB_Database
-->

If you look at your user table, do you see lastactivity as an index?