Database error in vBulletin :

[calorie]

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$config['SlaveServer']['usepconnect'] = 0;

My guess is probably, so recheck your config file for the following:

$config['MasterServer']['servername']
$config['MasterServer']['port']
$config['MasterServer']['username']
$config['MasterServer']['password']

And since you probably aren't using a 'slave' have the following;

$config['SlaveServer']['servername'] = '';
$config['SlaveServer']['port'] = 3306;
$config['SlaveServer']['username'] = '';
$config['SlaveServer']['password'] = '';
$config['SlaveServer']['usepconnect'] = 0;

And if all that is correct, do another thorough check of config, and if it's all correct, maybe ask your host what is wrong with their MySQL server.