[Marco van Herwaarden]

First make sure that if you have any members in your Tachy Goes Coventry, it only contain userid's and not usernames.

If that is not the cause, please remove any hacks and disable your plugins, then see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php

define('DISABLE_HOOKS', true);

If so, reupload all the original vB non-image files (except install.php). Make sure you upload these in ASCII format and overwrite the ones on the server.