I get the following error message while updating a Drupal site
According to http://drupal.org/node/1489394 some users experienced this problem when updating their rules module.  It looks like you'll need to update your rules, entity and internationalization modules all need to be updated to the latest versions (7.x-1.5 for internationalization and 7.x-1.0-rc2 for entities).  If you already have, try only updating those modules, before attempting to update the rules module (you can disable the rules module.  This should also solve the problem, but I assume you're using it for something or other).

As BetaRide points out above, checking into your web logs may shed some additional light on the problem, in case the above doesn't solve the problem.